What significant military action occurred in Panama on December 20, 1989?

The significant military action that occurred in Panama on December 20, 1989, was known as Operation Just Cause. This operation was a U.S. invasion to depose Panamanian leader Manuel Noriega, who was accused of drug trafficking and undermining democracy in Panama. The mission aimed to restore the democratically elected government and safeguard the lives of American citizens living in Panama.

Operation Just Cause involved thousands of U.S. troops and a variety of military resources. It emphasized a quick, decisive military action that focused on controlling key locations and taking down Noriega's military forces. The operation ultimately led to Noriega's capture and subsequent extradition to the United States to face charges.

The other options mentioned represent different military operations; for instance, Operation Urgent Fury was the U.S. invasion of Grenada in 1983, Operation Desert Storm was the combat phase of the Gulf War in 1991, and Operation Sovereign Frontier is not a well-known military action attributed to the timeline of U.S. military history. These differences highlight why Operation Just Cause is the correct answer for the events of December 20, 1989, in Panama.