Operation Fairfax was a joint counterinsurgency/pacification operation conducted by the II Field Force, Vietnam and the Army of the Republic of Vietnam (ARVN) in Gia Định Province, near Saigon lasting from November 1966 to 15 December 1967.

In October 1966, U.S. intelligence estimated that the Viet Cong (VC) in Gia Định Province, fielded eight battalions, each of 200-300 members known collectively as the 165A "Capital Liberation" Regiment. In Bình Chánh District the VC had overrun a police station and repeatedly cut roads in the area. Concerned by the deteriorating security situation around the capital, in November 1966 COMUSMACV General William Westmoreland proposed a joint U.S./ARVN security operation to General Cao Văn Viên Chief of the Joint General Staff (JGS). The operational plan would pair U.S. and ARVN Battalions to conduct security operations in Bình Chánh, Nhà Bè and Thủ Đức districts around Saigon. They would operate mostly at night, minimizing disruption to the civilian population and confronting the VC when they were most active. The units would set up population control checkpoints in coordination with the police and establish joint military/police security centers in each district to centralize the collection of intelligence.

II Field Force, Vietnam would be responsible for the U.S. participation taking a battalion from the 1st Infantry Division and two from the 25th Infantry Division. The ARVN units would come from III Corps. It was expected that by February 1967 the Vietnamese would be able to take responsibility for the entire operation and U.S. units could be deployed elsewhere.

The operation commenced in December and by the end of the month had claimed 235 VC killed or captured as compared to 15 in November, however despite a general improvement in security ambushes actually increased during this period.

In January 1967, General Westmoreland ordered that the 199th Infantry Brigade commanded by Brigadier general Charles Ryder take over as the dedicated unit responsible for the operation. In February VC killed or captured increased to over 300.

In March, Ryder was promoted and replaced as commander of the 199th Brigade by Brigadier general John F. Freund, Westmoreland's former assistant and MACV training director who had close relationships with Viên and National Police Chief General Nguyễn Ngọc Loan.

In March and April, security in the operational area appeared to improve, but this was regarded as illusory while the South Vietnamese Government continued to delay reforms and public works creating apathy among the populace who were open to the VC political agenda. Intelligence operations were having limited success in identifying and eliminating the VC political infrastructure.

In April the JGS charged that ARVN units performing security duties in the operation were helping local authorities and absentee landlords collect retroactive land rents and taxes from peasant tenants in formerly insecure areas. Viên and his chief of staff, Nguyen Van Vy, termed the practice illegal, if only because the owners themselves had paid no taxes for the period concerned. But the ramifications were obviously much deeper. If the practice was indeed widespread the Vietnamese farmers must have seen the returning government troops as oppressors, rather than liberators, and the Saigon regime that they represented as only concerned with the interests of a privileged minority. The entire concept of employing regular troops in this manner was thus open to question, and may explain why it was so easy for the VC to move men and supplies into the Saigon area for a major offensive scheduled for early 1968.