It took place from 22 January through 18 March 1969 and involved a sweep of the People's Army of Vietnam (PAVN)–dominated A Shau and Song Đa Krông Valleys by the 9th Marine Regiment.

[1] The 56 days of combat were a tactical success but did not stop the overall flow of North Vietnamese men and matériel into South Vietnam.

This "line" was a combination of infantry units and ground sensors devised to stop PAVN infiltration into South Vietnam across the border and along the Ho Chi Minh Trail.

He had noted that the manning of the bases and the defensive posture they developed was contrary to the aggressive style of fighting that Marines favor.

[2]: 16–8 In early 1969, intelligence reports indicated there had been a large PAVN build-up in the A Shau and Đa Krông Valleys.

[2]: 27–8 The operation, named Operation Dawson River South was to comprise 3 distinct phases: first was the southern movement of the 9th Marines and supporting units into mutually supporting firebases near the objective area, second was a period of intensive patrolling around the firebases and finally, the Regiment would attack into the PAVN base areas.

[2]: 34 On 2 February Firebase Cunningham was hit by 30-40 rounds of PAVN 122mm artillery fire from Laos resulting in five Marines killed.

On 5 February as Company G withdrew from Hill 1175 they were ambushed resulting in five Marines killed and 18 wounded, while only two PAVN bodies were found.

[2]: 35–6 On 10 February, Company H, 2/9 Marines captured a large cache of ammunition, weapons and equipment while on patrol five kilometers northwest of FSB Cunningham.

[2]: 46 As the Marines approached the Laotian border and in response to the artillery attack on Cunningham, Davis had sent requests up the chain of command to get permission to enter Laos.

This led to a redirection of MACV-SOG's Operation Prairie Fire to conduct reconnaissance near Base Area 611 in Laos.

"[2]: 41 On 21 February, Winecoff received a message from Colonel Barrow, 9th Marines Commanding Officer, to set up an ambush along Route 922.

Winecoff had not wanted the ambush sprung on one truck or soldier, realizing that eventually a bigger target would come down the road.

The Marines poured small arms and automatic weapons fire on the three vehicles, the forward observer alerted the artillery, and rounds bracketed the company position.

Morale was low because the Marines were tired after several days of patrolling, additionally, they did not want to leave the resupplies that included 60 mm mortar ammunition and C-rations.

Later that day a company patrol was ambushed by an estimated 15 PAVN troops who were dug in fortified bunkers and fighting holes.

For the record, all of the dead were listed as being killed in Quảng Trị Province, South Vietnam and for political reasons no reference was made about being in Laos.

As 3/9 Marines withdrew to Firebase Cunningham on 3 March they were ambushed by a PAVN force and PFC Alfred M. Wilson would be posthumously awarded the Medal of Honor for his actions during the firefight.

[2]: 50 Marine losses were 130 killed and 932 wounded, in return, the Marines reported 1,617 PAVN killed, the discovery of 500 tons of arms and munitions including 16 artillery pieces and 73 antiaircraft guns and denial of the valley as a PAVN staging area for the duration of the operation.

[7] In 1971, the operation to clear Highway 9 from Đông Hà Combat Base to the Laotian border was named Operation Dewey Canyon II in an attempt to misdirect enemy attention towards the A Shau Valley instead of Tchepone, the actual objective of the combined campaign.

In April 1971, the Vietnam Veterans Against the War organized a protest rally in Washington, D.C., and named it Operation Dewey Canyon III.

[8] This article incorporates public domain material from websites or documents of the United States Marine Corps.