When Napoleon overreached himself by usurping the Burbon throne of Spain in early 1808, the resulting Spanish resentment and the brutal suppression of the Dos di Mayo Rebellion in Madrid turned Spain, a long-term enemy of Britain, into an ally. Sir Arthur Wellesley’s expedition to South America was promptly cancelled and redirected to the Peninsular.
Eventually landing in Portugal at the mouth of the Rio Mondego, the army found itself with little transport, but by keeping close to the coast and with logistical support from the Royal Navy, Wellesley set off south to confront Marshal Junot and liberate Lisbon. The first clash of arms was at Obidos and, days later, at the Combat of Roliça, Wellesley achieved his first victory in battle against the willey General Delaborde.
With word that reinforcements were expected to arrive at Maceira Bay, the army was deployed at Vimiero to protect their landing. Meanwhile, Junot had assembled his army and decided to attack at Vimiero. In a short but hard-fought battle, Wellesley was victorious, however an opportunity for total defeat of the French was squandered in succession by generals Burrard and Dalrymple, who instead accepted an armistice. This eventually became the notorious Convention of Cintra.