Why did the USSR invade afghanistan?

Question: Why did the USSR invade afghanistan!?
Best Answer - Chosen by Asker:

A counter insurgent move!. The two nations had been closely allied for decades and the soviets had been in the country as military advisers!. The mujaheddin rebels we causing problems for the government and the Soviets were actually invited in to Afghanistan!. The Soviets were merely protecting their communist friends!.Www@QuestionHome@Com

The democratically elected Communist Government of Afghanistan was on the verge of collapse, and "asked" it's ally to come and help with those pesky mujhadeen!.Www@QuestionHome@Com