From the S&S archives:
U.S. shelled V.C. across Cambodian border

SAIGON — U.S. troops fired into Cambodia to crush a Viet Cong attack during action last weekend, it was revealed Tuesday.

It was the first time military officials have confirmed that U.S. soldiers fired across the border into the Republic of Vietnam's self-styled "neutral" neighbor.

There have been reports in the past of similar attacks, but spokesmen have declined to acknowledge any border violation until now.

The action opened when a battalion of 1st Inf. Div. troops, sweeping along the Cai Bac river about 60 miles northwest of Saigon, came under heavy mortar and machine gun fire from the opposite bank.

The Cai Bac forms the boundary between South Vietnam and Cambodia in that area.

The infantrymen ducked for cover and called in what was described as a "heavy volume" of artillery fire to blast the guerrilla position.

Artillery rounds raked the thick jungle across the river until the Viet Cong guns fell silent.

The number of casualties among the guerrillas was unknown. U.S. casualties in the action, which was part of Operation Birmingham, were light.

No U.S. ground troops crossed the river into Cambodian territory, the spokesman said.

U.S. officers in the field have long had authority to open fire on enemy forces firing from across the border.

The battalion commander who ordered the artillery barrage was following his prerogative "to take whatever means are at his disposal to protect his troops," according to the spokesman.

Questioned on the makeup of the enemy force, the U.S. spokesman said he "presumed" they were Viet Cong and not Cambodian.

It is known that South Vietnamese rebels use Cambodia both as a staging area and as a refuge when under attack.