<p>During the night of 29/30 March, heavy and mechanized North Vietnamese units drove into South Vietnam, moving simultaneously south across the DMZ and east from Laos in a powerful offensive. The RVNA fell back in disarray, and the defenses were left almost entirely to the air strength of the RVNAF and the Americans. It became a fierce fight, drawing on all available aircraft in the theater plus some deployed from other USAF command areas.
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<p>On 8 May, Nixon called a halt to the peace negotiations then underway in Paris, and authorized the Air Force to proceed with Operation Linebacker. Strikes were permitted again above the 20th parallel, and against a mix of new and old targets. Haiphong harbor and other northern ports were to be mined, and a naval blockade was established. The war had entered on a new phase, and there were developments in its technology that were to make things somewhat easier for the tactical strike forces headed north.
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<p>One of these was the maturing of the laser-directed and electro-optical guided bombs. These precision guided munitions (PGM) were extremely effective, and the kill probability of these 'smart' bombs became almost unity, instead of the low fractional values that had characterized the earlier iron 'dumb' bombs.
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<p>Improvements in electronic warfare aided immeasurably. Better 'Wild Weasel' equipment in newer aircraft was available, and the countermeasures pods for fighters and fighter-bombers were more versatile and effective. The EB-66 jammers had been refitted with the latest gear, and some big EC-135s lent their powerful jamming capabilities to shield the strikes north. Chaff corridors were laid down by fast-moving F-4s using new types of dispensers.
