Why did the Soviets invade Afghanistan?

In December 1979, in the midst of the Cold War, the Soviet 40th Army invaded Afghanistan in order to prop up the communist government of the People’s Democratic Party of Afghanistan (PDPA) against a growing insurgency.  At the time, the United States had been making headway in the Middle East at Moscow’s expense, successfully courting Egypt, Israel, Saudi Arabia, Pakistan, and others. The Soviet Union feared the loss of its communist proxy in Afghanistan.

Explanation:

(I search it on Internet.)

because of the war the westeners befriended japan and each other advance.