3 ways America have divided? And 3 ways the country is divided today?

Ways it was divided:  Indeed, it would be hard to argue that the country is more divided now than it was during the Civil War and in fact, the familiar proscription against discussing politics and religion in general company appeared at least as far back as a book of etiquette entitled Hill’s Manual of Social and Business Forms published in 1879. And it could certainly be said that our current political unrest has nothing on the 1960s. Consequently, part of today’s apparent divisiveness appears to be a matter of short-sighted perspective

Ways it is divided today: Noting the relationship between negative campaigning and press coverage, the study authors wrote that “negative (but not positive) messages are recycled ad infinitum by journalists who seek conflict and controversy above all else” and that “technology has facilitated citizens’ ability to seek out information sources they find agreeable and tune out others that prove dissonant.” In other words, campaign muckraking appears to increase affect polarization, while press coverage of that conflict fans the flame of confirmation bias.

"Can the United States Become More United Again?": If there is hope for Americans to move forward together as a country, it lies in the promise that democracy can bring those from different cultures and disparate viewpoints to the same table where they can work collaboratively based on a larger shared identity and the greater common good. Much of what we know, or think we know, about political polarization is based on polls and surveys that force respondents into dichotomous choices that may fail to capture the complexity of how people actually feel. In reality, many partisans have more nuanced views than one might assume, just as apparent “moderates” might have extreme opinions that are inconsistent with a unified party platform and therefore "cancel each other out"

Hope this helps! Good luck! I added the last one just as a bonus answer!

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d.)   when the us left nato after the end of the cold war and they had to evolve to focus on more humanitarian measures with the smaller budget