Which action taken by president andrew johnson triggered his impeachment trial?

President Andrew Johnson was impeached primarily because: (1) he fired the Secretary of War Edwin Stanton and (2) ignored the Tenure of Office Act. Johnson dismissed Stanton mainly because of the former's opposition in his Reconstruction policies. And although Stanton argued his protection due to the Tenure of Office Act which prevents a president from removing appointed officials without the approval of Congress, Johnson ignored it.