Without diverging too far, many countries use unit swapping and common training opportunities. I was once in an op with Kiwi Army soldiers and CA National Guard. Fun training exercise. Great troops and interesting people.
I have also seen around town Brittish Soldiers last year. I noticed over a few days there were enough of them wandering around this one place near where I work. I asked them and they said they were doing joint exercises with the Marines locally. Always great keeping relations up.