Nationalism seems to be resurging worldwide, and it's scary. WWIII is certainly evitable, but only if we are diligent about it.
We need more free trade and a common labour market. I actually agree with the EU's doctrine of the 'four freedoms.' The more economically interdependent states are, the less likely they will go to war with one another.
Now I know some will counter that a single labour market is not the same as immigration in that all it means is you don't need a work visa so it's not the same as permanent residency or citizenship, and they would have a point.
However, for the sake of world peace and security, I do believe we need at least one of the two. I personally am both pro-immigration AND for a single labour market. However, should people insist immigrants are just 'freeloaders,' then at least accept a single labour market. You can't say workers are freeloaders.
More open borders means more international friendships, more international marriages, etc. And that reduces the likelihood of war between states.