Venice's long decline started in the 15th century, when it first made an unsuccessful attempt to hold Thessalonica against the Ottomans (1423–1430). It also sent ships to help defend Constantinople against the besieging Turks (1453). After Constantinople fell to Sultan Mehmet II, he declared war on Venice. The war lasted thirty years and cost Venice much of its eastern Mediterranean possessions. Next, Christopher Columbus discovered the New World. Then Portugal found a sea route to India, destroying Venice's land route monopoly. France, England and the Dutch Republic followed them. Venice's oared galleys were at a disadvantage when it came to traversing the great oceans, and therefore Venice was left behind in the race for colonies.