The exact words of the United States Code, Title 18, Sections 2340, 2340A, and 2340B are shown below. As you read these words and consider the pictures and other public evidence from Abu Ghraib and other prisons, you will see: first, that numerous United States Military personnel did commit the crime of torture (as defined below), on a systematic and widespread scale, not confined to the rogue actions of a few individuals; second, that the very highest levels of the Bush Administration, including the President himself, must have planned and/or authorized these activities, and thus are guilty of the high crime of "conspiracy to commit torture," an impeachable offense.
Further, you will see that the entire chain of command - starting from the Bush-Cheney-Rumsfeld level, then going down through the military organizations and the CIA, down to the individual torturers - must have also been members of this conspiracy to commit torture, to the extent that they worked together to make it happen.
Here are the words of the Law . . .
Section 2340. Definitions
As used in this chapter -
(1) ''torture'' means an act committed by a person acting under the color of law specifically intended to inflict severe physical or mental pain or suffering (other than pain or suffering incidental to lawful sanctions) upon another person within his custody or physical control;
(2) ''severe mental pain or suffering'' means the prolonged mental harm caused by or resulting from -
(A) the intentional infliction or threatened infliction of severe physical pain or suffering;
(B) the administration or application, or threatened administration or application, of mind-altering substances or other procedures calculated to disrupt profoundly the senses or the personality;
(C) the threat of imminent death; or
(D) the threat that another person will imminently be subjected to death, severe physical pain or suffering, or the administration or application of mind-altering substances or other procedures calculated to disrupt profoundly the senses or personality; and
(3) ''United States'' includes all areas under the jurisdiction of the United States including any of the places described in sections 5 and 7 of this title and section 46501(2) of title 49.
Section 2340A. Torture
(b) Jurisdiction. - There is jurisdiction over the activity prohibited in subsection (a) if -
(1) the alleged offender is a national of the United States; or
(2) the alleged offender is present in the United States, irrespective of the nationality of the victim or alleged offender.
(c) Conspiracy. - A person who conspires to commit an offense under this section shall be subject to the same penalties (other than the penalty of death) as the penalties prescribed for the offense, the commission of which was the object of the conspiracy.
Section 2340B. Exclusive remedies
Nothing in this chapter shall be construed as precluding the application of State or local laws on the same subject, nor shall anything in this chapter be construed as creating any substantive or procedural right enforceable by law by any party in any civil proceeding.
Source: United States Code, as found at http://www.findlaw.com/casecode/uscode.html/ and other locations.
There are also other anti-torture laws which the United States is currently breaking, because of Bush's policies, and some of these surely could provide other grounds for impeachment of Bush and Cheney. But the law shown above seems to provide the clearest and most obvious grounds, because of its precise definition of torture and its specific prohibition of conspiracy to commit torture. An excellent summary of all United States and International Laws against torture is located at http://www.hrw.org/english/docs/2004/05/24/usint8614.htm As you read these laws, you will see for yourself how disgustingly far the Bush Administration has pulled us away from the civilized norms of human behavior.
After Bush and Cheney are impeached and removed from office, the Speaker of The House Of representatives - at that time - is next in line to become President, under existing U. S. law. (See http://bensguide.gpo.gov/9-12/government/national/succession.html) So prior to the actual removals from office, the House could select whomever it chooses to become Speaker. It is not Constitutionally required that the Speaker of the House be a Member of the House. So the House could elect whomever it chooses to become Speaker, who would then become President after Bush and Cheney are removed.