It's hard for Americans to remember all the times our side has militarily pushed around other sides. But the Russians remember, so it's worth it to recall how similar Croatia in 1995 and Georgia in 2008 look to the Kremlin.
Slavic Orthodox Serbia is a sort of mini-Russia that was at the core of a sort of mini-Soviet Union called Yugoslavia. So, Russians paid a lot of attention to what happened to Serbia in the 1990s, just as they did in 1914 when the Czar destroyed his dynasty in defense of Serbia.
Also, parts of the U.S. government didn't tell other parts what they were up to. The U.S. Ambassador to Croatia, for example, appears to have been sincerely appalled by the huge ethnic cleansing by Croatia, but he wasn't kept in the loop of planning the offensive. Or something.