While they received very limited material help from the North Vietnamese at the time (the Hanoi government had no incentive to overthrow Sihanouk, since it was satisfied with his continued "neutrality"), they were able to shelter their forces in areas controlled by PAVN/VC troops.

To accomplish this, President Lyndon B. Johnson authorized covert cross-border reconnaissance operations conducted by the secret Studies and Observations Group in order to gather intelligence on PAVN/VC activities in the border regions (Project Vesuvius).

[7]: 56–57  Continued unrest spurred Prime Minister/Defense Minister Lon Nol to close the port of Sihanoukville to communist supplies and to issue an ultimatum on March 12 to the North Vietnamese to withdraw their forces from Cambodia within 72 hours.

The prince, outraged that his "modus vivendi" with the communists had been disturbed, immediately arranged for a trip to Moscow and Beijing in an attempt to gain their agreement to apply pressure on Hanoi to restrain its forces in Cambodia.

[4]: 90 National Security Advisor Henry Kissinger wrote in his memoirs that "historians rarely do justice to the psychological stress on a policy-maker", noting that by early 1970 President Nixon was feeling very much besieged and inclined to lash out against a world he was believed was plotting his downfall.

[10]: 607 In particular, Nixon believed that a spectacular military action that would prove "we are still serious about our commitment in Vietnam" might force the North Vietnamese to conclude the Paris peace talks in a manner satisfactory to American interests.

[10]: 607  The first withdrawal of 1969 had led to an increase in PAVN/VC activities in the Saigon area, and Abrams had warned Nixon that to pull out another 150,000 troops without eliminating the PAVN/VC bases over the border in Cambodia would create an untenable military situation.

[10]: 606  Sihanouk left Moscow for Beijing, where he was greeted warmly by Zhou Enlai, who assured him that China still recognized him as the legitimate leader of Cambodia, and would back his efforts at restoration.

[11]: 561  By late March 1970, Cambodia had descended into anarchy as Karnow noted: "Rival Cambodian gangs were hacking each other to pieces, in some instances celebrating their prowess by eating the hearts and livers of their victims.

"[10]: 607 The North Vietnamese response was swift; they began directly supplying large amounts of weapons and advisors to the Khmer Rouge and Cambodia plunged into civil war.

[10]: 606  Even before the supply conduit through Sihanoukville was shut down, PAVN had begun expanding its logistical system from southeastern Laos (the Ho Chi Minh trail) into northeastern Cambodia.

[11]: 559  The CIA began to fly in arms for the Lon Nol regime, through the Secretary of State William P. Rogers told the media about Cambodia on March 23, 1970 "We don't anticipate that any request will be made".

[17] In early-April South Vietnamese Vice President Nguyễn Cao Kỳ twice visited Lon Nol in Phnom Penh for secret meetings to reestablish diplomatic relations between the two countries and agree on military cooperation.

The U.S. responded immediately, delivering 6,000 captured AK-47 rifles to the FANK and transporting 3–4,000 ethnic Cambodian Civilian Irregular Defense Group program (CIDG) troops to Phnom Penh.

Cao passed on these discussions to South Vietnamese President Nguyễn Văn Thiệu who verbally ordered the JGS to instruct ARVN III Corps to liaise with MACV for operations in Cambodia.

[5]: 152 The joint U.S./ARVN campaign would begin on May 1 with the stated goals of reducing allied casualties in South Vietnam, assuring the continued withdrawal of U.S. forces, and enhancing the U.S./Saigon government position at the peace negotiations in Paris.

[11]: 566  He announced his decision to launch American forces into Cambodia with the special objective of capturing COSVN, "the headquarters of the entire communist military operation in South Vietnam.

[20]: 178  Surrounded, they waited until nightfall and then with security provided by the 7th Division they broke out of the encirclement and fled north to unite with the COSVN in Kratie Province in what would come to be known as the "Escape of the Provisional Revolutionary Government".

[20]: 180 Years later Trương would recall just how "Close [South Vietnamese] were to annihilating or capturing the core of the Southern resistance – elite units of our frontline fighters along with the civilian and much of the military leadership".

[5]: 172 [2]: 56 Phase II of the operation began with the arrival of elements of IV Corps, consisting of the 9th Infantry Division, five armored cavalry squadrons and one Ranger group.

On 1 May a tape of Nixon's announcement of the incursion was played for Abrams, who according to Lewis Sorley "must have cringed" when he heard the President state that the capture of the headquarters was one of the major objectives of the operation.

[1]: 158  Due to increasing political and domestic turbulence in the U.S., Nixon issued a directive on 7 May limiting the distance and duration of U.S. operations to a depth of 30 kilometers (19 mi) and setting a deadline of 30 June for the withdrawal of all U.S. forces to South Vietnam.

Lon Nol had requested that the ARVN help in the retaking of Kampong Speu, a town along Route 4 southwest of Phnom Penh and 90 miles (140 km) inside Cambodia.

On 23 June the division moved to Đức Cơ Camp and was organized into four task forces which would then advance west along Route 19 to Ba Kev, protected by U.S. air cavalry units.

[2]: 100–105 The FANK units at Labang Siek would then move 35 km (22 mi) east along Route 19 to Ba Kev and would then be flown or trucked to Đức Cơ across the border to South Vietnam.

The role of the United States Air Force (USAF) in the planning for the incursion itself was minimal at best, in part to preserve the secrecy of Menu which was then considered an overture to the thrust across the border.

[1]: 149–151  Regardless, due to the ability of U.S. logisticians to innovate and improvise, supplies of food, water, ammunition, and spare parts arrived at their destinations without any shortages hampering combat operations and the communications system, although complicated, functioned well enough during the short duration of U.S.

[9]: 276 In June 1970 Thiệu met with Lon Nol, Prince Sirik Matak and Cheng Heng at Neak Loeung where the ARVN had established an operational base.

[2]John Shaw and other historians, military and civilian, have based the conclusions of their work on the incursion on the premise that the North Vietnamese logistical system in Cambodia had been so badly damaged that it was rendered ineffective.

[6]: 262–263  Regardless, the PAVN's Group 559 successfully countered these efforts through camouflage tactics and the construction of thousands of kilometers of "bypass" roads to avoid choke points that frequently came under enemy attack.