In 1808 French forces invaded Portugal trying to attempt to halt trade with Britain, turning Spain into a satellite state in the process. The British responded by dispatching a force under Sir Arthur Wellesley which captured Lisbon. Napoleon dispatched increasing forces into the Iberian Peninsula, which became the key battleground between the two nations. Allied with Spanish and Portuguese forces, the British inflicted a number of defeats on the French, confronted with a new kind of warfare called "guerrilla" which led Napoleon to brand it the "Spanish Ulcer".