Short answers, pulled from the story.
The Peninsular War escalated in 1808 when Napoleon occupied Spain, a former ally, after it had helped him invade Portugal in 1807. Napoleon forced the abdications of both King Ferdinand VII and his father Charles IV at Bayonne on the 5th of May 1808, then installed his brother Joseph Bonaparte on the Spanish throne. Most Spaniards rejected French rule and rose in revolt within weeks.
Napoleon called the conflict the "Spanish ulcer" because it was a drain on French resources that would not heal. Despite winning many individual battles, French forces were constantly harassed by Spanish guerrillas, their supply lines were severed, and more than 200,000 of their 350,000 soldiers in Spain were tied down guarding communications rather than fighting. The war allowed the rest of Europe to challenge Napoleon while his army was bogged down in Iberia.
The Battle of Bailén in 1808 was the first open-field defeat of a Napoleonic army on a European battlefield. The Spanish army in Andalusia defeated the French force under Pierre Dupont de l'Etang, who surrendered his entire army corps, a loss of around 24,000 troops. The defeat shocked Europe, demonstrated that Napoleon's armies could be beaten, and inspired Austria to challenge France in the War of the Fifth Coalition.
The Lines of Torres Vedras were three strong defensive lines of mutually supporting forts, blockhouses, redoubts, and fortified artillery positions built near Lisbon under Wellington's orders and the supervision of engineer Richard Fletcher. Construction began in the autumn of 1809, and the lines were finished just in time to stop Marshal Masséna's invasion force of around 65,000 men in 1810. A scorched-earth zone in front of the lines stripped the surrounding countryside of food, forage, and shelter, starving Masséna's army until he retreated in March 1811 having lost 25,000 men.
At the Battle of Vitoria on the 21st of June 1813, Wellington's army of 57,000 British, 16,000 Portuguese, and 8,000 Spanish defeated King Joseph Bonaparte's 65,000-man force. Wellington attacked with four columns from south, west, and north while a fourth column cut across the French rear, forcing a rout. The French abandoned all their artillery and King Joseph's personal baggage train, and never recovered control of Spain.
The war destroyed the social and economic fabric of both Spain and Portugal, and the years of fighting ushered in civil wars between liberal and absolutist factions. Revolts in Spanish America, partly inspired by the disruption of Spanish authority, led to the independence of all of Spain's colonies on the American mainland. The Cortes of Cádiz, which governed besieged Spain during the war, produced the Spanish Constitution of 1812, which became a cornerstone of European liberalism.