It was Ironhide who led the counteroffensive against the Decepticons at Mission City - well before Optimus Prime got on the scene. Ironhide even worked with Lennox's plan - "Get to the roof Sam, we will protect you!"

When Prime arrive, he basically followed on from Lennox and Ironhide's plan that was already in motion. Prime was well needed reinforcements for the Autobot-Human alliance, but it was Lennox and Ironhide who'd initially spearheaded that operation.