After destroying GDI's Philadelphia, taking the most important port in the region, and crippling their air support, the last remaining objective was securing Washington DC. Remaining GDI forces in the city had rallied on its outskirts, awaiting further orders and reinforcements, which were not coming. Nod command decided to engage them before they can have a chance to launch a counter offensive and threaten Nod's progress. Command of this mission was given to the Legendary Insurgent.
You begin with a small outpost in the south-west and a handful of infantry. Immediately construct a war factory to be able to build vehicles and gain access to an MCV. Then, set up an Operations center to gain the Black Hand's assistance and start training them alongside militant rocket squads. Then, building-building, start clearing GDI forces on this side of the highway.
Since you are limited to just one Tiberium field, plus whatever resources the silos spill when destroyed, use forces at your disposal wisely. The urban setting allows for maneuvering between buildings - if the enemy does not have direct line of sight, he cannot engage (unless he's a watchtower or similar). Garrison infantry in buildings as needed and slowly take the city inch by inch with flame tanks supported by attack bikes. The subway system is of great help, if you can maintain it long enough to send infantry diversionary attacks inside.
With GDI still flailing about trying to get reorganised, Nod overran Blue zone B-2 within a couple of days. However Kane called the Insurgent back from DC and gave a less competent Nod commander the task to deliver the coup de grace - destroying the Pentagon. In what would appear to be a tactical error, a GDI commander would manage to defend the last command hub and organise a counterattack that would eventually drive Nod out of Washington DC altogether.