(The first quote is from a right-wing source, which is why it speaks of "saving" Vietnam from Communism. Just read it for the facts and ignore the spin.)
http://www.aim.org/media_monitor/A3420_0_2_0_C/On May 10, 1972, Conyers introduced an impeachment resolution against Nixon, charging him with "high crimes and misdemeanors." These had to do with Nixon’s decision to order U.S. military retaliation against North Vietnam over a Communist offensive against the south. For the first time in the war, Nixon ordered the use of American B-52 bombers. They bombed Hanoi, the capital of North Vietnam.
To Conyers, a critic of the U.S. effort to save Vietnam from Communism, the bombing was a violation of both domestic and international law, including the United Nations Charter. Years later, when President Reagan ordered U.S. military troops to liberate communist-controlled Grenada, Conyers was at it again, introducing a resolution of impeachment that accused Reagan of violating international law.
http://www.cnn.com/ALLPOLITICS/time/1998/09/21/conyers.htmlAs a brash backbencher on the House Judiciary Committee in the early 1970s, Michigan Representative John Conyers was enough of an irritant to the Nixon White House to earn 13th place on the Administration's original enemies list. "Coming on fast. Emerging as a leading black anti-Nixon spokesman," presidential counsel John Dean noted next to Conyers' name. In May 1972, Conyers introduced a resolution on the House floor demanding that Nixon be removed from office for his conduct of the Vietnam War. The measure went nowhere, but Conyers kept at it for the next two years, and when the Judiciary Committee finally voted in favor of impeaching Nixon, Conyers relished his vindication. "Impeachment is difficult, and it is painful," Conyers said at the time, "but the courage to do what must be done is the price of remaining free."