The U.S. Air Force became interested in combat aircraft for use in low-intensity conflicts in the early 1960s. To some extent this was due to U.S. President Kennedy's interest but conflict in Southeast Asia was heating up as well.
U.S. Air Force combat aircraft were generally designed for a high-end nuclear war with the Soviet Union. During the Vietnam era they were adapted for use with conventional weapons. But there were also a number of systems developed for counter-insurgency wars. Among the most numerous such USAF aircraft was the A-37 Dragonfly or "Super Tweet" -- an adaptation of the USAF's small T-37 Tweet primary jet trainer to conduct ground attack operations.
The A-37 first flew in 1964 and 577 were built through 1975. A large number of these (250+) were provided to the Republic of Vietnam Air Force, but the A-37 was also operated by USAF units.
Early A-37As were succeeded by upgraded A-37Bs. The dual controls of the T-37 were retained but they were most often flown with a single pilot. Later an observer was added and the resulting OA-37B became the forward air controller of choice over South Vietnam.
When South Vietnam fell to the Communists in 1975, many A-37s/OA-37s were evacuated, but dozens fell into the hands of the victors. They operated these for some years before a lack of spare parts finally led to retirement of the type from Vietnamese service.
After Vietnam, the U.S. provided OA-37Bs to a number of Air National Guard units for service as forward air controllers. The OA-37s lasted until A-10 Warthogs finally replaced them in the 1980s.
A number of other small air forces in Latin America and Asia used A-37s and OA-37s and a few of these are still in service today.
https://en.wikipedia.org/wiki/Cessna_A-37_Dragonfly
A note on photos: Most of the photos show aircraft with 4-8 external fuel tanks. For short-range or short-duration missions, there were plenty of pylons to mount bombs and rockets. I was unable to find a decent photo of a RVNAF A-37.