WASHINGTON — A civilian appeals court on Monday vacated two convictions of a former aide to terrorist mastermind Osama bin Laden.In a split decision, the U.S. Court of Appeals for the District of Columbia Circuit concluded a military commission lacked authority to convict Ali Hamza Ahmad Suliman al Bahlul of two out of three charges.Military prosecutors charged Bahlul with conspiracy to commit war crimes, providing material support for terrorism and solicitation of others to commit war crimes. A military commission convicted him of all three crimes and sentenced him to life imprisonment.In the decision Monday, the appellate court rejected Bahlul’s challenge to the conspiracy charge but vacated the other two.“Solicitation of others to commit war crimes is plainly not an offense traditionally triable by military commission,” Judge Karen LeCraft Henderson wrote, adding that “the government offers little domestic precedent to support the notion that material support or a sufficiently analogous offense has historically been triable by military commission.”Bahlul had argued that the military commission could only consider actions that were recognized under the international law of war when committed. As part of its complicated ruling, the appellate court did not agree, stating that the 2006 law establishing military commissions “is unambiguous in its intent to authorize retroactive prosecution for the crimes enumerated in the statute—regardless of their pre-existing law-of-war status.”The complicated ruling included three lengthy concurring-and-dissenting opinions, as well as an unusual separate opinion in which Henderson explained how she might have ruled otherwise but for a legal concession made by the Justice Department. All told, the various opinions spanned 150 pages.“Today’s ruling is a reminder that a military commission prosecution or conviction can unravel at any time,” the Center for Constitutional Rights declared in a statement.The Center represents Australian David Hicks, convicted of a sole count of material support by a military commission. Hicks’ appeal seeking to have his conviction overturned has been stayed pending the D.C. Circuit decision.Bahlul is a native of Yemen. In the late 1990s, he traveled to Afghanistan to join al Qaeda.Bahlul, according to the appellate court’s ruling, served as a personal assistant to bin Laden, produced propaganda videos for al Qaeda and assisted with preparations for the attacks of Sept. 11, 2001. He admitted to creating an at-times cartoonish laptop computer recruiting video for the terror group al-Qaida. It glorified the October 2000 attack on the USS Cole warship off Yemen in which 17 U.S. sailors diedThree months after 9/11, Bahlul was captured in Pakistan and transferred to the United States Naval Base at Guantanamo Bay, Cuba. Bahlul, the appellate court reported, “flatly refused to participate in the military commission proceedings and instructed his trial counsel not to present a substantive defense.”Bahlul was known to shout “boycott, boycott, boycott” during Guantánamo court proceedings, mounted no defense at his trial. The appellate court’s 4-3 decision could result in a revised sentence for Bahlul. Carol Rosenberg of the Miami Herald also contributed to this report.