Does the United States have a duty to fight for freedom in neighboring countries?

Absolutely NOT - what right has the US to interfere in the politics of other countries? Example - funded the Taliban in Afghan against the USSR, pressured Pakistan to oust Musharraf despite him being pro American, because it suited them. There are stacks of others - the US needs to sort itself out - abuse of human rights - death penalty, Gitmo, poor unable to access healthcare, freedom of speech but not when you are protesting against the wealthy - there are lots of others! American 'freedom' is an abomination of the word freedom!

No, the US does not have a "duty" to fight for freedom in other countries - except, those countries with whom the US has made a treaty obligation to that effect.

For example, the US and UK have a mutual defense treaty. Should another party threaten the liberty of one of the treaty participants, the other is obligated to provide defense assistance.

However, the US is not obligated to fight for freedom in Libya. There is no treaty obligation. Yet, the US may do so in pursuit of its own best interest - including the desire to see freedom expand.

I don't find it anywhere in the Constitution that we should.