Question

What was the significance of the Monroe Doctrine?

Answer 1

In 1823, the United States was less 50 years old and pretty much still proving it could last as a country. European powers were losing long held lands to independence movements throughout the Americas.
President Monroe's Secretary of State was John Quincy Adams, he wrote the policy. The policy was that any effort by foreign powers to interfere with these new countries would be considered an attack on the United States.
The idea was to support the independence of these new countries.
Revolutionaries like Simon Bolivar loved it. Most of Europe ignored it or thought it a joke. The US Navy had a hard time protecting itself. There really was no way for the US to enforce the idea.
But we had pretty much settled our differences with Great Britain. And they were the world's superpower at the time. So the Monroe Doctrine was enforced with British sea power and influence at first.
What's the significance of this? It's a first step towards the American idea that we have a responsibility to take care of the rest of the world. We are still figuring out exactly what that means.