The Serbian police (and I mean Serbian as in Serbia, not Serbian as in "composed solely of Serbs") never, ever, leaves the territory of the Republic -- under any circumstances.
The same can be said of the Armed Forces, who haven't been in Bosnia since the Spring of '92, when the old federation finally ceased to exist.
Who, then, stayed in Bosnia after the Spring of 1992? Well, all those Serbs whose hometowns were within the borders of the then Socialist Republic of Bosnia and Hertsegovina did. They formed the nucleus of the future VRS/BSA.
Serbia did not participate in the war(s) of Yugoslav Succession with its own weaponry and/or troops in any way, shape or form.
As for supporting and aiding ITS OWN PEOPLE, Serbia stands guilty as charged. What's so wrong about Serbia supporting Serbs is beyond me. I don't see you complaining about Iran and Saudi Arabia supporting the Bosnian Muslims, or Al-Qaeda supporting the Kosovo Albanian terrorists.
Nikola Pashich (Prime Minister of Serbia in the early XX century) was right -- we should've never agreed to the Kingdom of Serbs, Croats and Slovenes (Yugoslavia) after our victory in WWI. We should've taken what was being offered to us by our Allies -- England, France and the US -- and left Slovenia and half of Croatia to whomever wanted to take them in.
Perhaps you're unaware of the connection between Arkan and the MUP, or any of the other myriad of ties between Serbia and the personnel active in Croatia or Bosnia during the wars in those places? If so, how do you manage to keep yourself in the dark so completely?
In short, either get a clue, or shut up.
As to Serbia participating in the wars in Croatia and Bosnia through means other than the MUP, the Serbian armed forces in Croatia and Bosnia received their logistics and pay from Belgrade, and Serbia even went so far as to put a press gang out in Serbia to help man military formations in Croata and Bosnia.
Milosevic ran the wars in Croatia and Bosnia from Belgrade. To argue to the contrary is simply foolish.