Infantry are units that are generally large in number and vulnerable to most weapons. They tend to be least vulnerable against armor-piercing weapons, like those on tanks, but are vulnerable to actually being run over by said tanks in most cases. They are lightly-armed in comparison to most vehicles, being limited in how much they can carry.
There are several kinds of infantry:
Second World War
Rifle Infantry, basic soldiers armed with assault rifles. Allied ones are supposedly armed with M16s and Soviet ones with AK-47s. Effective against other infantry. Vulnerable to anti-infantry weapons, themselves, tanks, fire weapons, artillery, and more-or-less everything else. In excessively large groups, however, they have been known to overrun whatever gets in their way, except Concrete Walls.
Engineers, lightly armored and unarmed infantry units able to instantly repair friendly buildings...or capture the opponent's. Capturing a structure requires that its hitpoints be "in the red" (reduced to less than 34% of maximum HP.) If Engineers infiltrate a structure with more HP than that, it loses HP equal to 33% of maximum HP.
Rocket Soldiers, infantry armed with man-portable guided missile launchers. Effective against tanks, some aircraft, some defenses, and walls. Weak against other infantry due to armor-piercing rockets. Average against Yaks (which can kill a group of Rocket Soldiers before any of them can fire a second time, but which are armored aircraft weak to the rockets)
Medics, unarmed infantry who can heal other infantry. Otherwise unremarkable.
Spies, sneaky Allied infantry units without any armor or weapons...but still dangerous. They disguise as a Rifle Infantry unit, then proceed to infiltrate a structure and gather information. Soviet and Allied infiltrations are different in effect: Allied -> Radar Dome = Spy Info radar screen (counts enemy units;) Soviet -> Radar = Can see what that player sees; Allied -> Silo/Ore Refinery = Select it and see the bar showing how full it is; Soviet -> Silo/Ore Refinery = Select it to see how much money (as a number) is in it. Other structures' infiltration shows you (if selected) what it is currently producing. As always with Spies, BEWARE OF ATTACK DOGS!
Thieves, close relatives (though seemingly undisguised) of the Spy. They infiltrate Ore Refineries and Silos, then proceed to steal half of whatever it contains. Unarmed. Still BEWARE OF ATTACK DOGS!
Grenadiers, Soviet infantry armed with hand thrown grenades. The cameo suggests a Fragmentation Grenade, but the effects imply Incendiary (such as a White Phosphorous grenade.) Exceptionally effective against groups of infantry, walls, and buildings. They can also lob their grenades over walls. Keep them spread out, though; like Flamethrowers, when they are killed, they explode and can cause chain reactions destroying whole groups of them at once. Also beware of Anti-Personnel mines, which toast groups of these guys for breakfast.
Flamethrower Infantry, Soviet pyromaniacs. Their name essentially says it all: "We have flamethrowers. Infantry, buildings and vehicles beware. We explode when we die. We have a short range." The only other thing you need to know about Flamethrowers is that if they get too close to a base, it goes down. Fast. But they need to be spread out, like Grenadiers. And they have projectile-flamethrowers, not the particle-stream ones used later on.
Attack Dogs, specially trained canine killing machines. Literally chews up and spits out infantry units before they know what happened. Spies, Thieves, Infiltrators, and stray cats, beware of the Attack Dog. Do not step on the yard, do not pet the dog, do not pass Go, and do not collect 200$. Avoid Tanya, vehicles, buildings, aircraft and ships at all costs with Dogs; only Tanya of these is vulnerable, and she can kill most of a moderately large group of them before they reach her.
Third World War
Conscripts, Conscripts are tough well armed frontline infantry whilst being cheaper than their counterparts the GI they can't deploy but have much tougher armor whilst having the same speed and roughly equal damage
Flak Troopers, similarly armored to the Conscripts but tougher and equipped with man-portable Flak Cannons for anti aircraft and infantry use.
Tesla Troopers, Soviet soldiers in large, Tesla-powered suits of armor. They are difficult to kill and carry Tesla weapons that are effective against infantry and vehicles, but weak against buildings.
Engineers, brainy specialists in structural capture and repair. They are unarmed and able to defuse Crazy Ivan's explosive charges.
Crazy Ivans, nearly (if not completely) insane soldiers who are willing to plant powerful explosives on anything or anyone, friend or foe. These bombs detonate after a time, and may be defused by an Engineer. (who is needed to detect an enemy CI bomb on your own units)
Desolators, Soviet unique units armed with radiation cannons. They are tremendously effective against infantry and very heavily armored, immune to radiation, and capable of irradiating an area as though a small nuclear weapon had gone off.
Terrorists, also Soviet unique units. They are suicidal infantry who, though apparently sane, are completely willing to simply walk into an Allied base and blow themselves to pieces for the Motherland.
Psi-Corps Trooper, Soviet specialists not normally buildable. These soldiers are members of the Psychic Corps (and so are lost to the Soviet Union during the Psychic Dominator crisis) able to use weaker versions of Yuri Clones' abilities, including mind-control and area-effect psiwaves.
Attack Dogs, like Engineers, are universally available to the Allies and Soviets. They are instantly lethal to all infantry units, including heroes. They may also detect disguised Spies. However, they are tremendously vulnerable to the lightest of vehicles, as they cannot damage anything at all mechanized. Terror Drones and anti-infantry vehicles are the bane of Attack Dogs. So are other Attack Dogs.
GIs are Allied infantry comparable to the Rifle Infantry of GWII and Soviet Conscripts. They are much better trained than their Soviet counterparts, but lack armor. (Conscripts are cheaper and harder to kill.) However, to offset this, they are better armed; they may deploy themselves into a sort of temporary foxhole and deploy a heavy machine gun at the same time, with longer range and more damage, and a higher rate of fire. They also take less damage in this state.
Guardian GIs are GIs with more armor. They deploy into a metallic foxhole, rather than a sandbagged one; they also pack anti-tank/anti-air missile launchers that they may use when deployed.
Engineers, well trained Allied specialist support units, can capture enemy buildings and repair friendly ones, civilian structures, and Bridges. They also have the unique ability to spot and defuse Crazy Ivan's bombs. And are not armed.
Attack Dogs are the Allied counterpart to the Soviet unit of the same name and are, aside from breed and Cheering animation, essentially identical to the Soviet ones.
SEALs are Allied commandos similar in capability to Tanya. They are strong swimmers, and make use of submachine guns and C4. Lots and lots of C4. They are capable of planting C4 on ships and buildings, destroying them in one, big, focused explosion. They tend to be extremely weak against small, fast hard targets, such as Terror Drones.
Rocketeers are Allied soldiers with jet-packs. They "take the high road" and carry two automatic weapons of some kind mounted in the jet-pack's armrests. They need never touch land in an entire battle and so have a tremendous line of sight. Also, since they are infantry, armor-piercing anti-aircraft weapons, such as Patriots or Guardian GI missiles, are less effective against them. ALL Flak and Gattling equipment is deadly to Rocketeers, particularly Flak Tracks and Gattling Tanks.
Chrono Legionnaires are Allied special-forces troops armed with a weapon capable of completely erasing any trace of an object's existence from history, or at least teleporting the object out of the known universe ("Never existed...") and equipped with a Chronosphere backpack allowing them to teleport around. They could not, however, walk and have extremely light armor.
Chrono Commandos are the last word in Allied special-forces troops. They may be built when an Allied barracks is present and an Allied battle lab has been spied upon. They are essentially SEALs with the Chronosphere backpacks, but are not amphibious.
Spies are Allied, well, spies. They disguise themselves as enemy infantry units and proceed to infiltrate an enemy building, playing some dirty trick or another, determined by what they infiltrated. BEWARE OF ATTACK DOGS!
Psi Commandos are more or less Psi Corps troopers that have been armed with C4 as well. They may be built after a Soviet battle lab has been infiltrated, in RA2; in Yuri's Revenge, infiltration of a Yuri battle lab was needed. (in YR, infiltrating a Soviet battle lab, regardless of side, allowed Chrono Ivan.)
Snipers are unique units for the Allies. With greater attack range, the Sniper can fire his rifle on enemy infantry in one shot. As of the Virus, this other Sniper can only fire their rifle filled with deadly toxins inside which causes enemy infantry to explode and die in action. The Virus has also less attack range than their rivals as SAS forces can use the Sniper to pick off enemy soldiers. They were used in a Sniper civil war against Yuri's Viruses during Operation: Power Play in Seattle of the United States of America.
Initiate are more powerful than either the Allied GI or the Soviet Conscript, being able to start fires with only their minds. They were very effective against infantry and structures, as well as vehicles, if encoutered in large numbers or garrisoned in a building.
Engineer which is really no different from the regular engineers, aside from its camo and dialogues.
Brute was Yuri's tank hunter. It was a huge mutant who could smash almost anything with its powerful fists. They were specially designed to take out enemy tanks, because tank weapons were almost useless against Brutes. Brutes were immune to dogs, because their veins were embedded too deeply in their bodies for a dog to tear out - not to mention, canines were simply to scared of them to attack. They were also immune to Yuri's mind control, because their psychology differed so greatly from a normal human.
Virus was Yuri's sniper unit. Much like the British Sniper it could kill infantry from a great range. While the Sniper had greater attack range, the Virus was armed with some deadly toxins that could quickly kill infantry units, causing them to explode in a cloud of toxic virual fumes. Even structures and tanks would be damaged, when coming into contact with these clouds, but infantry would explode and die, creating a sort of chain reaction.
Yuri Prime are Yuri ride a hover platform. He can mind-control any base defenses, structures, infantries, tanks, or ships. Yuri Prime's secondary weapon is much more useful than his clones, as his psychic shockwave will not harm friendly infantry, only damaging enemy infantry. It also has a shorter cooldown time than his clones.
War of the Three Powers
Peacekeepers, Allied basic infantry units. Armed with shotguns whose effectiveness increases as range decreases- against a single unit. Against groups, moderate ranges (which do not dissipate damage) allow a single shot to injure, if not kill, several enemy troopers. With their Riot Shields up, they are nearly immune to damage from the front, and should they close in to a garrisoned structure, they clear it from the inside. Great in close quarters urban battle.
Javelin Soldiers, big troops armed with Javelin missile launchers. Able to let the missiles target themselves, for a faster lock-on (useful against aircraft and fast vehicles, such as Sickles) or target the missiles manually, with a laser-guidance mode, in which the missiles are launched faster and hit harder, more effective against tanks and ships that stray too close. Vulnerable to faster moving infantry, such as Peacekeepers, Conscripts, Imperial Warriors, etc.
Attack Dogs, mostly the same as always. They are reliable canine forces, but do not make good pets. With the ability to amplify their bark enough to stun a full-grown human, and sometimes even the Soviet Bears, and to swim to protect better against infiltrators, these dogs are even more dangerous than before. Spies beware. However, dogs are still vulnerable to other dogs, and all vehicles and aircraft. And only useful against infantry while swimming, as, other than Infantry, Bears, Dogs, and Dolphins, there are no living naval units.
Allied Engineers, brainy civilian specialists, good with technology, and tools. Able to set up a medical tent to help heal nearby Allied infantry units. They, of course, still can capture and repair buildings, and repair bridges. Unarmed.
Spies, men of stealth serving the Allied cause. Masters of disguise, able to convincingly depict a swimming Conscript or even other Spies. Able to bribe enemy units nearby with their large "discretionary bank accounts". Also taught how to swim, just to make Soviet guards' jobs more difficult.
Cryo Legionaires, Allied soldiers who are the best of the best. They are armed with weapons that freeze targets close in; they have jetpacks to traverse battlefields quickly, their boots freeze water instantly (making them amphibious just by walking normally) and they can damage units by landing on them from a jetpack jump. They are also very, very expensive.
Conscripts, surprisingly zealous Soviet people "taught" how to fire their ADK-45 assault rifles by a short instructional video. They lack significant training, but their Molotov Cocktails can clear out garrisoned buildings from range, one unit at a time, and also make good weapons against bases and some vehicles. Their assault rifles are less powerful than the shotguns of Peacekeepers, but are effective at longer ranges and fire much faster. Good against other infantry, but best in numbers.
Flak Troopers, convicts and volunteers alike doing "Community Service." They carry large Flak Cannons, which they fire to great effect against the Union's enemies too cowardly to fight on the ground and those enemies on the ground. They are very effective against aircraft, and fairly effective against vehicles. They also have magnetic mines, useful for breaching walls, destroying buildings, demolishing vehicles.
War Bears, armored and well-trained grizzly bears. They can swim, and are death to any infiltrator unwary of their presence, if somewhat slow and bulky. They may, like Attack Dogs, amplify their roar to stun enemies. Great against intruders.
Soviet Combat Engineers, enlisted men of science armed with recycled, otherwise obsolete revolvers. Able to capture buildings, repair friendly buildings and bridges, use a collapsible rubber dinghy to traverse water and build useful defenses called Battle Bunkers.
Tesla Troopers, Soviet infantry in powered armor. These soldiers are all shock and awe, due to their Tesla weaponry and slow movement. Their armor reliably turns most bullets, but is unfortunately magnetic enough for Magnetic Satellites to pick it up. It is also vulnerable to the Electromagnetic Pulse, the Trooper's special ability, which stops nearby vehicles (and Tesla Troopers) cold. Effective against nearly everything, but expensive. Also able to power-up Tesla Coils.
Desolators, Soviet troops in similar powered armor. They carry large amounts of chemical weaponry (rumored to be wastes from poorly-maintained Super Reactors) that is extremely effective against all other infantry. They may also use their special ability, and become deadly against even tanks, as the chemicals corrode metals and react with the Desolators' normal weapons.
Imperial Warriors, fanatics with kinetic carbines and beam katanas. Effective against infantry to a similar degree as Conscripts, when using the carbine. With swords out, they can slaughter any person unlucky enough to still be in the area, and run at frightening speeds.
Tankbusters, troopers armed with plasma-beam-cutters. They effectively slice apart enemy vehicles, but are unable to hit aircraft and are almost useless against infantry. Able to burrow into Spider Holes for protection, or to set up an ambush. In fact, such ambushes appear to have been set up long before the Empire even entered the war.
Shinobi, Imperial operatives best described as "ninjas." Able to swim and capable of wreaking havoc once they enter a building. Armed with shuriken and swords, they are doom to any infantry who get in the way of their activities. Perfect for infiltrating guarded areas and assassinations.
Rocket Angels, Imperial military-trained women with flight packs, missile swarm pods, and paralysis whips. They can take to the skies. Frighteningly effective against most vehicles, except Bullfrogs and IFVs. The Paralysis Whip makes a surface unit unable to respond or move, allowing the others to go by and ignore it, or reinforcements to eliminate it. They are also good at taking on aircraft, especially as they fly themselves, but need not reload. Just keep them away from air defences.
Archer Maidens, soldiers similar to their flying sisters-in-arms, the Rocket Angels. They carry seemingly obsolete bows though the arrow is rocket assisted and has a large, explosive warhead. They are deadly against threats on the ground, or in the air.