This is a key aspect of a unitary system of government -The national government can revoke only certain decisions made at lower levels of government.

- The national government controls all foreign and economic policy Certain powers are delegated to lower levels of government states , cities, etc. -and the national government has little or no authority in those areas.

- The national government has supreme power over lower levels of government , which carry out the policies of the national government .

The national government has supreme power over lower levels of government, which carry out the policies of the national government .

Explanation:

A unitary form of government is a form of government that gives all major powers to the central government. There is no power sharing among the local and national government rather all the policies and decisions are made by the national government.

A key aspect of a unitary system of government is that national government has supreme power over lower levels of government, which carry out the policies of the national government.

The local governments can implement and execute only those decisions and policies laid out by the national government. Counties like France, UK, Colombia, etc have a unitary system of government.

There were several events that contributed to the fall of the Soviet Union. The most important ones ( with the dates included) are as follows:

December, 1979: Soviet troops invade Afghanistan to help the country's communist government to fight the guerrilla forces.January , 1981: Ronald Reagan becomes U.S. president. During his two terms he implemented a weapons policy that  the Soviet Union economy couldn't afford.March , 1985: Mikhail Gorbachev becomes president of the Soviet Union at 54 years old. He introduces glasnost (meaning openness) and perestroika (menaing reestructuring) in an effort to overcome economic and political problems.  February , 1989: The last Soviet soldier leaves Afghanistan. May , 1990: Boris Yeltsin is elected chairman of the Russian Soviet Federated Socialist Republic. (URSS) December , 1991: After surviving a coup, Mr. Gorbachev resigns as Soviet president, marking an end of the Soviet Union.