Which of president johnson’s actions led directly to his impeachment by congress?

He fired the Secretary of War in violation of the Tenure of Office Act.

Explanation:

Republican radical lawmakers twice tried the Supreme Court to intervene against President Andrew Johnson. Unable to succeed in these attempts, the same deputies found in the 1867 Tenure of Office Act, the prerogative to open a process of impeachment against Andrew Johnson in 1868. With only one year in force, that law provided that the president of the Republic could dismiss its secretaries of State without the Congress authorization.

On August 12, 1867, Jonhson had not observed the Tenure of Office Act and had removed the Secretary of War (named for Lincoln) Edwin Stanton. Stanton was allied with the radicals. His departure was the breach used for the impeachment process to begin.