Clyde Nelson is “not aware of any serious American violation of the Laws of War ever,” according to his 9/11 letter.
With Victor’s Justice prevailing, the U.S. isn’t often convicted of such crimes. The June 27, 1986 International Court of Justice conviction that U.S. financing, training and arming of the Nicaraguan Contras and its mining of Nicaragua’s harbors violated international law is the exception. The ICJ awarded $18 billion in reparations to Nicaragua. The U.S. hasn’t paid.
Most US violations of laws of war remain allegations, and there is a long list anyone can study.
In Korean, U.S. soldiers systematically killed as many as 400 civilian refugees over three days in July 1950 in the S. Korean village of No Gun Ri. Declassified documents found by a team of AP reporters in April 1998 showed that orders had been given to shoot civilians in combat areas. The team won the Pulitzer Prize for its investigation.
The U.S. sprayed about 19.5 million gallons of Agent Orange, Agent Blue, Agent White, phosphorus and other toxic chemicals over South Vietnam. The Chemical Weapons Convention prohibits the manufacture, stockpiling or use of such things.
In September 1970, a secret US Special Forces action in Laos named Operation Tailwind used nerve gas in over 20 attacks intended to kill American defectors. The Sarin gas allegations were confirmed by Adm. Thomas Moorer, Chairman of the Joint Chiefs in 1970. Maj. Gen. John K. Singlaub and Special Forces Lt. Robert Van Buskirk also corroborated reports of the gassing, which is barred by the Geneva Conventions. Reporters Peter Arnett and April Oliver stand by their June 1998 CNN and Time magazine reports, although both Time and CNN retracted the stories under pressure -- according to Arnett and Oliver -- from Henry Kissinger and Gen. Colin Powell.
The war on Vietnam involved countless atrocities, notably the B52 carpet bombing of villages in violation of Article 25 of the Hague Conventions which prohibits “attack or bombardment, by whatever means, of towns, villages, dwellings, or buildings which are undefended.” On the ground, the practice of U.S. Tiger Force soldiers in the Central Highlands in 1967 was commonplace. Our soldiers “tortured, mutilated and killed scores of unarmed civilians,” according to a series of articles by The Toledo Blade in October 2003. “Women and children were intentionally blown up in underground bunkers.… Prisoners were tortured and executed, their ears and scalps severed for souvenirs,” the paper reported.

US troops in Vietnam “cut off heads” -- John Kerry

Secretary of State John F. Kerry testified to Congress as a combat vet, in 1971, that U.S. troops “raped, cut off heads, taped wires from portable telephones to human genitals, razed villages in fashion reminiscent of Genghis Khan, shot cattle and dogs for fun, poisoned food stocks and generally ravaged the countryside….” Kerry said that at the Winter Soldier conference, “over 150 honorably discharged and many very highly decorated veterans testified to war crimes committed in Southeast Asia, not isolated incidents but crimes committed on a day-to-day basis with the full awareness of officers at all levels of command.”
In the 1991 Gulf War, between 400 and “thousands” of Iraqi soldiers who may have been surrendering were buried alive in trenches by U.S. Army plows while Bradly Fighting Vehicles “kept everybody’s head down” by directing heavy fire into the trenches.  
Between 1991 and 1995, U.S.-imposed sanctions against Iraq killed as many as 576,000 children, according to the UN Food and Agriculture Organization. In Sept. 1993, U.S. helicopters fired on women and children in Somalia, killing 100.
In May 1999, U.S.-piloted jets bombed Belgrade’s Chinese embassy, an undefended civilian target, and that April, U.S. jets destroyed a passenger train -- bombing it twice, the second time after circling back-- killing as many as 45. In May 1999, U.S. pilots fired four missiles into a combined nursing home and tuberculosis sanatorium in the village of Surdulica, killing 18 and wounding 43. The Geneva Conventions prohibit any bombing not justified by clear military necessity.
Sgt. Frank J. Ronghi was charged with rape, sodomy, indecent acts with a child and premeditated murder of an 11-year-old Albanian girl in Vitina, southern Kosovo in March 2000.
In Feb. 2010, the “killing for thrills” of Afghan civilians by Army soldiers involved cutting off fingers for “trophies,” and the soldiers videotaping themselves with severed heads of dead civilians. Spc. Jeremy Morlock was sentenced to 24 years in prison. U.S. Marines from a sniper platoon were shown desecrating corpses in a July 2011video that filmed them urinating on bodies of dead Afghan fighters. U.S. jets bombing Libya in August 2011, killed at least 34 civilians in the farming village of Majer, but NATO refuses to cooperate with international investigators who have demanded to know how and why it happened.

The torture of prisoners is a crime of war that the U.S. is justifiably quick to charge against enemies if it happens to our soldiers. But President Obama has admitted that “we tortured;” and a 560-page report on the subject by a panel loaded with Republicans found last April 13 that the US has practiced torture since 1991, and “the nation’s highest officials bear some responsibility for allowing and contributing to the spread of torture.”
Short lists of likely U.S. war crimes were published here Jan. 15, 2009 (Stop bombing Afghan villages) and May 17, 2012 (What’s NATO ever done?). But the Military Tribunal at Nuremberg characterized U.S. wars in Afghanistan and Iraq long before they started, declaring: “To initiate a war of aggression is not only an international crime; it is the supreme international crime, differing only from other war crimes in that it contains within itself the accumulate evil of the whole.”