Why was president andrew johnson charged with breaking the tenure of
office act?

Theodore R. Davis's illustration of President Johnson's impeachment trial in the Senate, published in Harper's Weekly.
The impeachment of Andrew Johnson was initiated on February 24, 1868, when the United States House of Representatives resolved to impeach Andrew Johnson, 17th president of the United States, for "high crimes and misdemeanors", which were detailed in 11 articles of impeachment. The primary charge against Johnson was violation of the Tenure of Office Act, passed by Congress in March 1867, over his veto. Specifically, he had removed from office Edwin M. Stanton, the Secretary of War—whom the Act was largely designed to protect—and attempted to replace him with Brevet Major General Lorenzo Thomas. (Earlier, while the Congress was not in session, Johnson had suspended Stanton and appointed General Ulysses S. Grant as Secretary of War ad interim.)

Johnson became the first American president to be impeached (the only one prior to Bill Clinton in 1999) on March 2–3, 1868, when the House formally adopted the articles of impeachment and forwarded them to the United States Senate for adjudication. The trial in the Senate began three days later, with Chief Justice Salmon P. Chase presiding. On May 16, the Senate failed to convict Johnson on one of the articles, with the 35–19 vote in favor of conviction falling short of the necessary two-thirds majority by a single vote. A ten-day recess was called before attempting to convict him on additional articles. The delay did not change the outcome, however, as on May 26, it failed to convict the president on two articles, both by the same margin; after which the trial was adjourned.

The impeachment and trial of Andrew Johnson had important political implications for the balance of federal legislative–executive power. It affirmed the principle that Congress should not remove a president from office simply because its members disagreed with his policies, style, or administration of duties. It also resulted in diminished presidential influence on public policy and overall governing power, fostering a system of governance which Woodrow Wilson referred to in the 1870s as "Congressional Government."[1]