Who was the first president to be impeached, and what was the reason?

The first president to be impeached was President Andrew Johnson, and the primary reason for his impeachment was his violation of the Tenure of Office Act. This law, passed in 1867, was designed to restrict the president's ability to remove certain officeholders without the Senate's approval. Johnson's attempt to dismiss Edwin M. Stanton, the Secretary of War, and appoint a successor without the Senate's consent was viewed as a direct challenge to the law and Congress's authority.

This event played a significant role in the tumultuous relationship between Johnson and the Republican-controlled Congress during the Reconstruction era. His impeachment reflected broader conflicts over the direction of post-Civil War reconstruction and the balance of power between Congress and the presidency. The process culminated in a trial in the Senate, where Johnson was acquitted by one vote, thus remaining in office.

The other options refer to different presidents associated with significant issues or scandals but do not pertain to impeachment. Grant, Nixon, and Tyler faced various challenges, but Johnson is distinctly noted as the first to be impeached under specific legal grounds.