A century and a half after his valiant death at the Battle of Gettysburg, a Union Army officer is being awarded the nation's highest military decoration, thanks to a decades-long campaign by his descendants and Civil War buffs.

The White House announced Tuesday that President Barack Obama approved the Medal of Honor for 1st Lt. Alonzo H. Cushing, who was killed standing his ground against Pickett's Charge during the pivotal, three-day Battle of Gettysburg.

Congress granted a special exemption last December for Cushing to receive the award posthumously since recommendations normally have to be made within two years of the act of heroism and the medal awarded within three years.

The White House also announced that Obama will award the medal in a ceremony on Sept. 15 to two Vietnam War soldiers who also received the congressional exemption – Army Command Sgt. Maj. Bennie G. Adkins and Army Spc. Donald P. Sloat. The medal is given to members of the Armed Forces who risk their own life in an act of great personal bravery.

Cushing was born in Delafield, Wisconsin, raised in Fredonia, New York, and buried at his alma mater, West Point, after being killed on July 3, 1863, at age 22. He commanded about 110 men and six cannons, defending the Union position on Cemetery Ridge against Pickett's Charge, a major Confederate thrust that could have turned the tide in the war. Cushing received a bullet wound in the head.

The fierce battle near the town of Gettysburg, Pennsylvania, resulted in more than 51,000 casualties. Confederate soldiers advanced into the Union fire but eventually retreated with massive losses. The South never recovered from the defeat. Four months later, President Abraham Lincoln memorialized the Union war dead in his Gettysburg Address.

During the battle, Cushing's small force stood their ground under a severe artillery bombardment as nearly 13,000 Confederate infantrymen waited to advance. Cushing was wounded, and his battery was left with two guns and no long-range ammunition. Historians say his stricken battery should have been withdrawn and replaced with reserve forces, but Cushing insisted on ordering his guns to the front lines on the last day of fighting.

"Refusing to evacuate to the rear despite his severe wounds, he directed the operation of his lone field piece continuing to fire in the face of the enemy," the White House said in its announcement. "With the rebels within 100 yards of his position, Cushing was shot and killed during this heroic stand. His actions made it possible for the Union Army to successfully repulse the Confederate assault."

More than 1,500 soldiers from the Civil War have received the Medal of Honor, most recently Cpl. Andrew Jackson Smith of Clinton, Ill., who was awarded the medal in 2001 by President Bill Clinton. It's not clear why Cushing never got one, but his descendants and admirers took up his cause in the late 1980s.

The Cushing name is prominent in Delafield, his birthplace in southeastern Wisconsin. A monument to Cushing and two of his brothers – Naval Cmdr. William Cushing and Army 1st Lt. Howard Cushing – stands at Cushing Memorial Park, where the town holds most of its Memorial Day celebrations.

Wisconsin lawmakers pushed through an amendment to a defense spending bill to award Cushing in 2010, but then-Sen. James Webb (D) of Virginia stripped it from the bill because he said it was impossible to go back 150 years to determine who should receive the award. Webb predicted it could open an endless series of claims and argued at the time, "The better wisdom would be for Congress to leave history alone."

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The award also will be given posthumously to Sloat, who was killed in action in Vietnam on Jan. 17, 1970, at age 20. Sloat, of Coweta, Oklahoma, picked up a live grenade triggered by a fellow soldier and used his own body to shield the blast and save his fellow soldiers.

Adkins, a veteran who served 22 years and has retired to Opelika, Alabama, plans to receive his medal in person. He was deployed three times to Vietnam with the Special Forces and is being awarded for actions in combat on his second tour, in 1966, when he ran wounded through enemy fire to drag wounded comrades to safety.