Between 1964 and early 1965 the Vietnamese had nothing to threaten American pilots in the air.
U.S aircraft flew at an altitude of 4−5 kilometers, and the Vietnamese anti-aircraft guns were unable to reach them.
However, after an S-75 shot down a U.S. F-4 Phantom aircraft, the U.S. bombers began to descend below three kilometers, below the minimum operational height of the Dvina.
This brought them within the reach of Vietnamese anti-aircraft guns.

On July 24, 1965, four U.S. Air Force McDonnell F-4C Phantoms took part in an airstrike against the Dien Ben Phu munitions storage depot and the Lang Chi munitions factory west of Hanoi.
One was shot down and three damaged by SA-2s.
This was the first time U.S. aircraft were attacked by SAMs
Two days later President Johnson gave the order to attack known SA-2 positions outside the 30-mile exclusion zone.
On the morning of July 27 48 F-105s participated in the strike, Operation Spring High.
But the Vietnamese knew U.S. aircraft were coming and set up many 23mm and 37mm anti-aircraft guns at the two SAM sites.
These anti-aircraft guns were lethal at close range; as a result the Vietnamese shot down six aircraft and more than half of the remaining U.S. aircraft suffered damage from ground fire.
Both SAM sites were devoid of missiles and equipment; the Vietnamese had substituted white-painted bundles of bamboo for the SA-2s.
Operation Spring High had destroyed two worthless targets for the loss of six aircraft and five pilots.

Between 1965 and 1966 the US developed countermeasures to the S-75 threat.
The Navy soon had the AGM-45 Shrike in service and mounted their first offensive strike on a site in October 1965.
The Air Force fitted B-66 bombers with powerful jammers (which blinded the early warning radars) and developed smaller jamming pods for fighters (which denied range information to enemy radars).
Later developments included the Wild Weasel aircraft, which were fitted with anti-radiation air-to-surface missile systems made to home in on the radar from the threat.
 This freed them to shoot the sites with Shrikes of their own.

The Soviets and Vietnamese were able to adapt to some of these tactics.
The USSR upgraded the radar several times to improve ECM (electronic counter measures) resistance.
They also introduced a passive guidance mode, whereby the tracking radar could lock on the jamming signal itself and guide missiles directly towards the jamming source.
This also meant the SAM site's tracking radar could be turned off, which prevented Shrikes from homing in on it.
Some new tactics were developed to combat the Shrike.
One of them was to point the radar to the side and then turn it off briefly.
Since the Shrike was a relatively primitive anti-radiation missile, it would follow the beam away from the radar and then simply crash when it lost the signal (after the radar was turned off).
SAM crews could briefly illuminate a hostile aircraft to see if the target was equipped with a Shrike.
If the aircraft fired a missile, the Shrike could be neutralized with the side-pointing technique without sacrificing any S-75s.
Another tactic was a "false launch" in which missile guidance signals were transmitted without a missile being launched.
This could distract enemy pilots, or even occasionally cause them to drop ordnance prematurely to lighten their aircraft enough to dodge the nonexistent missile.

At the same time, evasive maneuvers were used, and intensive bombardments of identified SAM firing positions were organized.
Under these conditions, camouflage and radio silence became especially important.
After combat launches, an anti-aircraft missile division was to leave the area immediately, otherwise it would be destroyed by a bombing attack.
Until December 1965, according to American data, eight S-75M systems were destroyed, although sometimes American aircraft bombed dummy positions equipped with decoy missiles made of bamboo.
Soviet and Vietnamese calculations claimed the destruction of 31 aircraft; the Americans acknowledged the loss of 13 aircraft.
According to the memoirs of
Between 1964 and early 1965 the Vietnamese had nothing to threaten American pilots in the air. U.S aircraft flew at an altitude of 4−5 kilometers, and the Vietnamese anti-aircraft guns were unable to reach them. However, after an S-75 shot down a U.S. F-4 Phantom aircraft, the U.S. bombers began to descend below three kilometers, below the minimum operational height of the Dvina. This brought them within the reach of Vietnamese anti-aircraft guns.
On July 24, 1965, four U.S. Air Force McDonnell F-4C Phantoms took part in an airstrike against the Dien Ben Phu munitions storage depot and the Lang Chi munitions factory west of Hanoi. One was shot down and three damaged by SA-2s. This was the first time U.S. aircraft were attacked by SAMs
Two days later President Johnson gave the order to attack known SA-2 positions outside the 30-mile exclusion zone. On the morning of July 27 48 F-105s participated in the strike, Operation Spring High. But the Vietnamese knew U.S. aircraft were coming and set up many 23mm and 37mm anti-aircraft guns at the two SAM sites. These anti-aircraft guns were lethal at close range; as a result the Vietnamese shot down six aircraft and more than half of the remaining U.S. aircraft suffered damage from ground fire. Both SAM sites were devoid of missiles and equipment; the Vietnamese had substituted white-painted bundles of bamboo for the SA-2s. Operation Spring High had destroyed two worthless targets for the loss of six aircraft and five pilots.
Between 1965 and 1966 the US developed countermeasures to the S-75 threat. The Navy soon had the AGM-45 Shrike in service and mounted their first offensive strike on a site in October 1965. The Air Force fitted B-66 bombers with powerful jammers (which blinded the early warning radars) and developed smaller jamming pods for fighters (which denied range information to enemy radars). Later developments included the Wild Weasel aircraft, which were fitted with anti-radiation air-to-surface missile systems made to home in on the radar from the threat.  This freed them to shoot the sites with Shrikes of their own.
The Soviets and Vietnamese were able to adapt to some of these tactics. The USSR upgraded the radar several times to improve ECM (electronic counter measures) resistance. They also introduced a passive guidance mode, whereby the tracking radar could lock on the jamming signal itself and guide missiles directly towards the jamming source. This also meant the SAM site's tracking radar could be turned off, which prevented Shrikes from homing in on it. Some new tactics were developed to combat the Shrike. One of them was to point the radar to the side and then turn it off briefly. Since the Shrike was a relatively primitive anti-radiation missile, it would follow the beam away from the radar and then simply crash when it lost the signal (after the radar was turned off). SAM crews could briefly illuminate a hostile aircraft to see if the target was equipped with a Shrike. If the aircraft fired a missile, the Shrike could be neutralized with the side-pointing technique without sacrificing any S-75s. Another tactic was a "false launch" in which missile guidance signals were transmitted without a missile being launched. This could distract enemy pilots, or even occasionally cause them to drop ordnance prematurely to lighten their aircraft enough to dodge the nonexistent missile.
At the same time, evasive maneuvers were used, and intensive bombardments of identified SAM firing positions were organized. Under these conditions, camouflage and radio silence became especially important. After combat launches, an anti-aircraft missile division was to leave the area immediately, otherwise it would be destroyed by a bombing attack. Until December 1965, according to American data, eight S-75M systems were destroyed, although sometimes American aircraft bombed dummy positions equipped with decoy missiles made of bamboo. Soviet and Vietnamese calculations claimed the destruction of 31 aircraft; the Americans acknowledged the loss of 13 aircraft. According to the memoirs of
S-75 Dvina