We are (likely) less than a century from colonizing another planet. When that happens, will we stop being so nationalistic? Should we?

You should take a look at The Moon Is a Harsh Mistress by Robert Heinlein, basically a retelling of the American Revolution in a lunar-colony setting. The politics [laws] of space aren't equipped to deal with all that right now. I'm more interested to see what happens when someone like Elon will be the first to Mars and he is not a government. If possession is 9/10ths of the law [the law that doesn't exist right now] well. Space X is extremely dependent on the U.S. Government like its LC39A launch pad in Florida leased from NASA... but given some decades of development and all those commercial satellite contracts Space X has going for them you could imagine a scenario where Space X is fully capable of running the Mars show soup to nuts with just federal oversight on things like airspace and launch windows.

Personally I think we should. The first nation to invest in the energy to get there should be the one to have the piece of land they occupy. Initial sustainment of a colony would take some serious LOGPAC operations.

Sir, for the most part I love the concept and idea of colonizing another planet......I believe in the beginning we will be in a way for himan beings are tribal in nature.... with that said though, i do see where the colonials of the new planet will eventually seek thier own nationalistic views for either being colonials or thier descendents being born on said planet. Sort of like when America was colonized and formed.