So there are apparently various magical communities throughout the world (as evidenced by the various magical schools and sports teams we see). We can assume these communities have their own governments. So my question is, why didn't these governments do anything once Voldemort took over? Wasn't Voldemort a threat to the entire magical world?
My question is somewhat similar to this "Why didn't the other schools help Hogwarts?" but I feel it is different enough.
Ignorance.
You will note that Wizarding Britain is largely ignorant of almost anything that goes on abroad. There's barely any mention of anything international, short of Quidditch, importing of cauldrons (Go Percy!) and Triwizard - which didn't happen for a couple hundred years.
What makes you think Fudge is in any way interested in, or has mental capacity, to understand political situation in Magical US? And chances are, the rest of MoM equivalents are run pretty much by the same incompetent Peter-Principle buffoons, ruling over equally apathetic populations.
Sovereignity.
Voldemort is strictly an internal issue in Britain. Other communities would need a valid reason to interfere, meaning a request from MoM.
First, Fudge wasn't even admitting Voldie is back. Then, Scrimgeour acted like he had stuff under control and clearly wouldn't admit weakness by asking for help. Then, Thicknesse was Voldemort's puppet. All-in all, nobody who'd ask for help.
Indifference.
Voldemort didn't affect anyone else directly. So nobody is on a humanitarian tear to do peacekeeping/nationbuilding/insert-muggle-20th-century-term operations. Don't forget that Wizarding World is basically stuck in 1600s, largely - politically as well as psychologically.
Your question can be answered in our own history. Why didn't the United States enter World War II sooner than it had, the reason was that at the time was that the Nazi weren't much of a problem to the U.S. It wasn't until the attack of Pearl Harbor in 1941 that caused the U.S. to help stop the growing problem. The same goes with Voldemort, other schools did help, such as Beauxbatons and Durmstrang did in fact help out since they were directly affected/effected by what was happening, but schools overseas like the American School of Wizardry or the Brazilian Wizarding School wouldn't be have had much involvement. So, unless something happened in the United States, like actually causing the New York Twin Towers to be destroyed (only an example), the United States would have stayed out of it.
