Short answers, pulled from the story.
The Peninsular War started in July 1807 when Emperor Napoleon Bonaparte issued orders to his Foreign Minister Charles Maurice de Talleyrand-Périgord demanding Portugal declare war on Britain. The conflict continued until 1814, with major battles occurring through June 1813.
French troops were sent under divisional general Jean-Andoche Junot who marched on Lisbon starting from the 12th of October 1807. Napoleon ordered this force known as the First Corps of Observation of the Gironde to occupy Portugal after Prince John of Braganza refused to join the Continental System against British trade.
A secret Treaty of Fontainebleau had been signed by Napoleon's marshal of the palace Géraud Duroc and Eugenio Izquierdo an agent for Manuel Godoy. This document proposed carving up Portugal into three entities including the Kingdom of Northern Lusitania and the Principality of the Algarves.
Bloody spontaneous fighting known as guerrilla literally little war broke out in much of Spain against the French as well as Ancien Régime officials. The term guerrilla came from this conflict and entered the English language through these events with key leaders like Juan Martín Díez known as El Empecinado troubling French troops across wide areas.
In August 1808 fifteen thousand British troops including the King's German Legion landed in Portugal under command of Lieutenant-General Arthur Wellesley. He drove back Henri François Delaborde's four thousand strong detachment at Roliça on the 17th of August and smashed Junot's main force of fourteen thousand men at Vimeiro.