No America (the US) is unusual in history. It's continent wide size, natural resources, and great distance from other continents means it doesn't have to rely on anyone. US allies, NATO, UN, etc. are just political conveniences, to promote US trade and world stability and peace.
If all US allies disappeared tomorrow, no one in the US would notice.
This 'has' happened before in history when the US practiced 'isolationism' and shut itself off from the rest of the world.
America's allies only benefit the allies and to some extent US business, America doesn't need the allies, but America does need world peace and stability....and that means allies.
Hi! I think there's no image of the chart but I'd like to answer about the Columbian exchange that might be the answer. The Columbian Exchange started was the widespread of trading of Americans and Afro-Asians and was followed by the voyage of Christopher Columbus to the Americas in 1492. They were exchanging plants, animals, technology, ideas, diseases and human populations.
Droughts were the biggest problem.
Hope this helps!
