The fall of tenochtitlán in 1521 was significant because

It was a major step in the big cultural ( and ethnic and religious) shift in the Americas: From the Indigenous cultures to European ones. The Aztecs and the Incas were the only empires strong enought to stand a chance against the conquerors, and when  they failed, the whole continent basically "stood open" for the colonizers, who would then encounter only limited resistance.

The 5th amendment allowed this to happen.