Why are many of the states that were mainly Republican 100 years ago more Democratic today and vice versa?

Mostly Political campaigning and geographical promises

Explanation: states will swing between parties depending on how much a presidential candidate will campaign in that state. For instance, Trump did a lot of campaigning in 2016 in Florida and turned it red, but in 2012 obama won Florida. As for Geographical promises a state will give electoral votes to the presidential candidate who will promise they will take care of the states best features, for example they will promise North Carolinians protection of their farms.

propriety is the answer. hope this !

explanation:

on april 6, 1917, the u.s. joined its allies--britain, france, and russia--to fight in world war i. under the command of major general john j. pershing, more than 2 million u.s. soldiers fought on battlefields in france.