Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Unhappy with this change of policy by the Portuguese government, Napoleon sent an army to invade Portugal. On 17 October 1807, 24,000 French troops under General Junot crossed the Pyrenees with Spanish cooperation and headed towards Portugal to enforce Napoleon's orders. This attack was the first step in what would eventually become the Peninsular War, a six-year struggle that significantly sapped French strength. Throughout the winter of 1808, French agents became increasingly involved in Spanish internal affairs, attempting to incite discord between members of the Spanish royal family. On 16 February 1808, secret French machinations finally materialized when Napoleon announced that he would intervene to mediate between the rival political factions in the country. Marshal Murat led 120,000 troops into Spain and the French arrived in Madrid on 24 March, where wild riots against the occupation erupted just a few weeks later. Napoleon appointed his brother, Joseph Bonaparte, as the new King of Spain in the summer of 1808. The appointment enraged a heavily religious and conservative Spanish population. Resistance to French aggression soon spread throughout the country. The shocking French defeat at the Battle of Bailén in July gave hope to Napoleon's enemies and partly persuaded the French emperor to intervene in person.
When did Napoleon's invasion of Portugal begin?