The events of the French Revolution, especially the regicide of Louis XVI and the Terror, made the rest of Europe's monarchs fear for their lives. The Portuguese monarchy, like others, took measures to prevent the infiltration of revolutionary propaganda into the kingdom. Maria I, who suffered nightmares and fits of melancholy, imagined that she was damned. In 1792 she turned the reigns of government over to her second son, Joã o, who was prince of Brazil. As the situation in France deteriorated, Portugal signed treaties of mutual assistance with Britain and Spain in 1793. In the same year, the Spanish army, reinforced by 6,000 Portuguese troops, attacked France across the Basque frontier. In 1794 the French launched a major counterattack, which forced the combined Spanish-Portuguese army to retreat from French territory. The French army reached the Ebro River and threatened Madrid.

In 1795 Spain made peace at Basel with France without consulting the Portuguese. Despite having fought with the Portuguese against France, the Spanish now allied themselves with the French and signed a secret treaty at San Idelfonso in 1800. In 1801 France and Spain sent the Portuguese an ultimatum threatening to invade Portugal unless it abandoned its alliance with Britain, closed its ports to the British and opened them to French and Spanish ships, and handed over one-quarter of its territory as a guarantee for Spanish territories held by Britain. The Portuguese refused to comply, and the Spanish marched into the Alentejo in May. After two weeks of fighting, the "War of the Oranges," as it is known, was concluded in 1801 at Badajoz. According to the terms of the peace treaty, Portugal agreed to close its ports to British shipping, granted commercial concessions to the French, paid an indemnity, and ceded Olivença to Spain.

When Napoleon became emperor in 1804, he renewed his struggle with Britain. The British declared a naval blockade of France, and, in retaliation, Napoleon decreed that all nations of Europe should break relations with Britain. Portugal declared itself neutral in the struggle. Napoleon ordered the Portuguese to close their ports to the British, which they were prepared to do if they could without breaking relations with their old ally. In October 1807, Napoleon signed a treaty with Spain at Fontainebleau, according to which France and Spain agreed to invade Portugal and partition the country, one-third going to France, one-third to Spain, and one-third to Spain's chief minister, Manuel de Godoy.

On November 17, 1807, an army of French and Spanish soldiers under the command of the French general Andoche Junot entered Portugal and marched on Lisbon. The British were in no position to defend their ally; consequently, the prince regent and the royal family left for Brazil. On November 27, Junot's army took control of Lisbon.

French occupation eventually sparked rebellions among the populace, and provisional juntas were organized in several cities. The junta in Porto, to which other local juntas finally pledged obedience, organized an army and, with British help, was able to defeat a strong French force at Lourinhã on August 21, 1808. After this defeat, the French opened negotiations with the Portuguese and signed the Convention of Sintra, which provided for the evacuation of Junot's forces. The government was placed in the hands of the juntas. In January 1809, the prince regent designated a British officer, William Carr Beresford, to reorganize the Portuguese army, granting him the rank of marshall and commander in chief.

In March 1809, French troops under the command of General Nicholas Soult invaded Portugal once again. Entering the country from Galicia, they occupied Chaves and marched on Porto. A combined Portuguese-British army, commanded by Sir Arthur Wellesley, pushed Soult back to Galicia and defeated another French army at Tavera in Spain, after which Wellesley was made the duke of Wellington.

The expulsion of Soult's forces gave the Anglo-Portuguese army time to prepare for Napoleon's third invasion, which was ordered in 1810. The third French army under the command of General André Masséna entered Portugal at Guarda and marched to Viseu. Because Wellington's forces held the main roads, Masséna took his army across the Buçaco Mountains and marched on Coimbra, which he sacked. Wellington withdrew his army southward, luring Masséna into positions he had prepared at Tôrres Vedras. Finding the positions impenetrable, Masséna, far from his source of supply and short of food, withdrew his forces. Wellington pursued Masséna and overtook him at Sabugal where his army was defeated. Masséna retreated from Portugal.

Data as of January 1993