The Costa Plan was an overall strategic plan for to bring the Portuguese Civil War to a quick end.
The idea of the plan was to quickly win the war by invading northern Spain and quickly come around to encircle the Royalist and armies and bring the war to an end before it begins.
Politically the major drawback of the plan involved violating Spanish neutrality. To counter this Gomes da Costa suggested invading Spain in the west beforehand with a small force so as to draw the Spanish armies away from the northern border. Eventually it was discovered that Spain had been sending aid to the Royalists. This information persuaded the Portuguese president, Manuel de Arriaga to attack. Although he did make some changes to the plan, mainly he wanted a larger force to invade Spain from the west and a smaller force to invade from the north. Unfortunately for Portugal, the British SIS (which had been formed in 1910) knew about the plan and decided to use the BEF to stop the Portuguese at Vigo.