Answer:
The main cause of the war, was the Assassination of Arch Duke Franz Ferdinand. The sinking of the Lusitania only cause the U.S. to enter and help the British and French.
Explanation:
The Portuguese King found out that Spain was sponsoring trips to the new world and me became very angry. Then, the King sent a threatening letter to the Catholic Monarchs and in those, he stated that by the Treaty of Alcáçovas, which was signed in 1479 and confirmed in 1481 with the papal bull Æterni regis, all lands at the south of the Canary Islands were granted to Portugal. Which caused them to go to the Pope, which then created a line to divide the new lands between those two countries, in a way to avoid a war that the Spanish most likely would lose.
In the end, Portugual was not pleased with the division, mostly because it prevented them to conquer India witch was a goal pursued by them for a long time. So they went to the Catholic Monarchs to renegotiate directly the position of the line. The treaty of Tordesillas was the diplomatic solution found by the Catholic Monarchs. Curiously, even though the treaty and the resulting line was negotiated without consulting the Pope, some sources call the division the "<em>Papal Line of Demarcation"</em>.
