I don't assume the US (merely using it as an example when we question the possibility of conspiracy theories being true).

Arafat could have been killed by other (more radical) Muslims, by the Israelis, by any number of people (though the method suggests a government was probably involved). Milosevic was likely killed by some western government, based on circumstance (being in custody at the Hague), and the fact that he revealed that people were trying to kill him before it happened...

<ignore reason=probably not true>There aren't many with the capability.</ignore> Polonium has no stable isotopes and is naturally so rare that the nuclear industry synthesises it from bisimuth using neutron beams in specialist nuclear reactors. <ignore reason=probably not true>Also, apart from triggers for nukes and poisoning dissidents, it has very few uses.</ignore>