Manfest Destiny was a political plank in the platform of the Democratic party developed in the 1830s and 1840s justifying the expansion of the United States into the west and even into Canada and Mexico.
The Democrats were trying to push the boundaries of the United States into both Mexico and Canada. In 1844 the Democrats wanted to expand into Canada and developed the campaign slogan "5440 or Fight" as part of their campaign for Manifest Destiny. 5440 referred to the 54th parallel in Canada. The campaign slogans died out when the people of the United States refused to extend into Canada or Mexico. In the 1850s the Republican party began and by 1860 the people of the United States elected a president far more concerned with internal politics of slavery than the external politics of expansion. Manifest Destiny was dead.
The Monroe Doctrine was a political policy developed in 1823 by the United States to end imperialism by European nations in the Americas.
For some reason the Monroe Doctrine is often associated with Manifest Destiny. I'm not sure why, probably by political propagandists trying to convince people that the United States is an expanding empire.
The Monroe Doctrine has been used to protect commercial investments by United States citizens.
Some people claim this is what happened with the Panama Canal. I'm not so sure about that, the canal has resulted in the ability to reduce shipping costs of Asian goods into South American, North American and the Caribbean as well as reducing the cost of shipping goods produced in these regions to Asia.
There have been other uses of the Monroe Doctrine that are questionable, but, the Panama Canal benefits the entire world.
I read a lot of different things and I realized that most people read primarily fiction and what non-fiction they do read is very limited to specific ideologies in specific areas. Rather than educating themselves broadly they educate themselves very narrowly.
This is not a problem with the United States, this is a problem with the world.
People become frustrated and they attack someone, anyone, to relieve their frustrations. It almost always ends up being that no matter how many times people revolt they always end up putting people in power who oppress them and then they revolt again.
If I had a choice there would be no governments. There would be no governments. People would live in small agricultural villages. Commerce would be conducted using sailing ships and caravans. Anyone who tried to establish any form of political control would instantly die.
That ain't going to happen. People want someone to tell them what to believe, what to do.