The answer is that The Soviet Union invaded Afghanistan to help its Communist government.
Although US was involved, it does not invaded the nation instead was offering help with the consent of Afghanistan government.
In 1979, the US helped the opposition faction in the Afghanistan to launch an armed uprising in order to overthrow the pro-Soviet government. The Soviet Union, worrying that it would lose control over Afghanistan, send troops and invaded it immediately.
As Afghanistan was one of the major oil supplier to US, once the region fell into USSR control, the oil supply would be greatly affected, thus it was resentful towards USSR's action of the invasion, causing their relations to deteriorate.
Therefore The Soviet Union invaded Afghanistan to help its Communist government is the answer.
Hope it helps!
