What invoked the principle of "checks and balances" during President Andrew Johnson's impeachment?

The principle of "checks and balances" was invoked during President Andrew Johnson's impeachment primarily because Congress challenged Johnson's authority, leading to a significant constitutional crisis. This situation arose from Johnson's conflicts with Congress over Reconstruction policies following the Civil War. He attempted to enforce his vision of reconstruction, which was at odds with the more punitive approach favored by many lawmakers.

His actions, including the violation of the Tenure of Office Act by attempting to remove Edwin Stanton, the Secretary of War, without congressional approval, prompted Congress to act. This created a situation where legislative checks were necessary to control the actions of the executive branch. The impeachment process itself is a clear manifestation of the checks and balances system outlined in the U.S. Constitution, which allows Congress to hold the president accountable for misconduct while safeguarding the separation of powers.

In this context, it wasn't the Supreme Court, public opinion, or foreign powers that prompted the invocation of checks and balances, but rather the direct challenge and subsequent impeachment by Congress, reflecting the role of legislative oversight in the federal government. This historical instance illustrates how the framers of the Constitution anticipated potential conflicts between branches of government and designed a system to address such conflicts through checks and balances.