Why did the Soviet leadership, which in March 1979 refused to send troops to Afghanistan, changed its mind in December?

Why did the Soviet leadership, which in March 1979 refused to send troops to Afghanistan, changed its mind in December? How is the change in the position of the leaders of the USSR related to the development of the missile crisis?

In March 1979, the Soviet leadership refused to send its troops to Afghanistan at the request of Taraki, due to the fact that it did not encourage the extremist steps of Afghan leaders. The Soviet leadership urged Taraki to pursue a more balanced policy. But when Taraki was overthrown and killed, and Hafizul Amin came to power, the Soviet leadership changed its mind (it was just December), because Amin was considered an unpredictable person, he could at any time reorient himself to the United States or China. In this case, a hostile state could arise at the borders of the USSR, which we would not want. That is why Soviet troops were nevertheless brought into Afghanistan. With the development of the missile crisis and the war in Afghanistan, the Cold War has resumed. In 1980-1982, the United States imposed a number of economic sanctions against the USSR. In 1983, R Reagan called the USSR an “Evil Empire” and called for its liquidation. Plus began installing new American missiles in Europe. In response to this, the General Secretary of the CPSU Central Committee Yu.V. Andropov abruptly stopped all negotiations with America and the world came to the brink of a third world war.