Twilight of War - The Airmen Series, #20

By the summer of 1973, US combat troops have withdrawn leaving only a handful of American advisors and CIA operatives to assist South Vietnam's struggle for survival. The country's fate hangs by a thread as a massive Communist invasion from the North looms. Abandoned by America, South Vietnam's dispirited generals cede the frontier, awaiting attack against the cities and the Mekong Delta's rice fields.
Unwilling to desert their allies, weathered CIA veterans Rene Granier and Tom Coyle hatch an audacious plan - assemble several teams of elite operatives to launch audacious strikes to shatter Communist momentum and demoralize the NVA troops. If their daring raids can shock Northern confidence and rekindle ARVN resistance, Saigon may still survive the coming onslaught. But failure to disrupt the Communist tide will ensure Vietnam's swift fall to a red dawn.
Outgunned, Granier and Coyle gamble everything on breaking the siege mentality before Saigon is swallowed by Vietnam's gathering twilight of war.