Should a past involvement in facilitating the illegal sale of weapons to an enemy regime in order to finance terrorists seeking to overthrow a democratically elected government and a central belief in the use of Machiavellian politics discount someone from a position of influence in our government? I would say without reservation: yes. President Bush, on the other hand, would disagree.
Michael Ledeen is one of the top foreign policy advisors of the Bush administration, being at one point (and to my knowledge may still be) the only full-time foreign advisor that Karl Rove consulted. Ledeen is a vocal proponent of regime change in the Middle East. He was also formerly directly involved in the Iran-Contra scandal.
The 2003 invasion of Iraq can be seen as a direct application of foreign policy doctrine developed by Ledeen, as one of the leading thinkers of the neoconservative movement, well before the 9/11 attacks, and it is important to note that the influential Ledeen has advocated not only invasion of Iraq, but also invasion and military dominance of Iran, Syria and Lebanon as an extension of the war on terror.
Something else that might cause a person concern over Ledeen's influence on this administration are his ties to fascism and fundamentalists such as Pat Robertson.