Civil wars always have foreigners involved for several reasons. Some will backup one side, such as the British backing up the loyalists, or the US backing up the Pakistani army against the Taliban. In those cases the reasons are political.

During the Cold War, the civil wars in Central America saw several countries involved. The US, the British, the Canadians, the French, the Germans, the Israelis, they all had interests there. The CIA provided training for the military and traded weapons and drugs, and for that reason the US had the finger pointed at it through the Cold War.

Today we have forgotten wars such as the civil war in the Soviet Union involving Adzerbaijanis and Armenians fighting over Nagorno-Karabak. In that war western powers and Iran backed and supplied the Adzerbaijanis, while the Russians backed the Armenians. Cold War politics and Islamic extremism fueled foreign involvement, and curiously, both the west and Iran backed the same side.

I find that in most civil wars, somebody is getting rich at the expense of the parties involved in that war. The IRA bought weapons, as did the Loyalists, and there were thirds parties that made money supplying them. It is not different in Afghanistan and Pakistann today. The Taliban, who fought against the Russians in the 1980s, are now buying Kalashnikov rifles Russian smugglers, while the Pakistani army is buying its weapons from the US. Then corrupt Pakistani army officers are selling American weapons to the Taliban. In the mean time who makes the money? Weapons manufacturers of course!