William Howard Taft IV

After the election of 2000, George W. Bush appointed Taft to serve as chief legal advisor to the United States Department of State under Secretary of State Colin Powell, with whom he was reportedly friends. This appointment was technically a significantly lower appointment than he had held in other administrations, but it permitted him to work with his wife, Julia Taft, a top State Department official in charge of refugees who also served during the Clinton administration.

While serving as Legal Adviser, Taft wrote two seminal law journal articles regarding the views of the United States on the legality of the use of military force. First, in connection with the decision of the International Court of Justice in the Oil Platforms case, Taft countered a series of propositions that the court appeared to accept regarding the principles governing the use of force. This included his conclusion that "There is no requirement in international law that a State exercising its right of self-defence must use the same degree or type of force used by the attacking State in its most recent attack. Rather, the proportionality of the measures taken in self-defence is to be judged according to the nature of the threat being addressed."[3] Second, Taft coauthored (with Todd F. Buchwald) an article in the Americal Journal of International Law that set forth the official United States Government view regarding the permissibility under international law of the use of force by the United States and Coalition forces during the 2003 invasion of Iraq. The article concluded that the preemptive use of force in Iraq is lawful where, as in Iraq, "it represents an episode in an ongoing broader conflict initiated--without question-- by the opponent and where, as here, it is consistent with the resolutions of the Security Council.[4]

In 2004, Taft's name surfaced as a dissenter concerning the policy of interrogation techniques for military detainees.[5] In a January 11, 2002, memo, Taft opposed Department of Justice lawyers to argue that the president could not "suspend" U.S. obligations to respect the Geneva Conventions and that a legal argument to do so was "legally flawed and procedurally impossible." This was also the position of Secretary Powell, who attempted to persuade Bush to reconsider. Alberto R. Gonzales, the White House counsel, subsequently advised Bush in a memo that Taft and Powell were wrong and the Justice Department's analysis was "definitive." Gonzales claimed terrorist attacks "require a new approach in our actions toward captured terrorists," and argued that if suspected terrorists had never respected the Geneva Conventions' human rights protections, the U.S. didn't need to do so.

After the re-election of President Bush, resignation of Colin Powell and appointment of Condoleezza Rice as secretary of state, Taft resigned to return to private practice, again at Fried, Frank, Harris, Shriver & Jacobson. Currently he is a visiting professor at Stanford Law School, having succeeded Allen Weiner as the Warren Christopher Professor of the Practice of International Law and Diplomacy in 2007.[6] In January 2009 he was named chair of the board of trustees for Freedom House, an independent watchdog organization that supports the expansion of freedom around the world.[7]

On September 12, 2006, Taft co-signed (along with 28 other retired military or defense department officials) a letter to the chairman and ranking member of the Senate Armed Services committee in which he stated his belief that the Bush Administration's attempt to redefine Common Article 3 of the Geneva Convention "poses a grave threat" to U.S. service members.

Taft is said to be one of the sources who told journalists David Corn and Michael Isikoff that former Deputy Secretary of State Richard Armitage was the source syndicated columnist Robert Novak had when he made public the fact that Valerie Wilson worked for the CIA. In a review of Corn's and Isikoff's book, Hubris: The Inside Story of Spin, Scandal, and the Selling of the Iraq War, Novak wrote: "I don't know precisely how Isikoff flushed out Armitage [as Novak's original source], but Hubris clearly points to two sources: Washington lobbyist Kenneth Duberstein, Armitage's political adviser, and William Taft IV, who was the State Department legal adviser when Armitage was deputy secretary."[8]

Though a staunch Republican, Taft opposed the 2016 presidential campaign of Donald Trump, declaring alongside 49 other Republican former national security officials that he would not vote for the candidate.[9] In 2020, Taft again rebuked his party, endorsing Democrat Joe Biden over Trump.[10]

In 2020, Taft, along with over 130 other former Republican national security officials, signed a statement that asserted that President Trump was unfit to serve another term, and "To that end, we are firmly convinced that it is in the best interest of our nation that Vice President Joe Biden be elected as the next President of the United States, and we will vote for him."[11]