First European Nation to Send Colonizers to the Americas: Spain (1492)

Correction: Christopher Columbus sailed in 1492 (not "the 1400s" generically), funded by Queen Isabella I of Castile and King Ferdinand II of Aragon—the unified Crown of Spain. He reached the Caribbean (Guanahaní, Hispaniola, Cuba), not mainland North America. Spain was the first European nation to launch sustained colonization in the Americas.