The south vietnamese invaded with american air support.
a. thailand
b. laos
c. cambodia

The correct answer is B, as the South Vietnamese invaded Laos with American air support, in the Operation Lam Son 719.

Operation Lam Son 719 was a military offensive developed by the South Vietnamese Army led by General Xuan Lam in command of 20,000 soldiers and with US air support that left the city of Khe Sanh in 1971, during the Vietnam War.  This mission was trying to cut by land the HoChi Minh Route that contributed important supplies and troops to the contingents that fought in the South.

The action, which involved an invasion of Laos by a breach of 35 km long and 25 km wide, failed in its attempt to suppress supplies and reinforcements sent from the north to the south of Vietnam through Laos.

lincoln presided over the union victory in the american civil war, which dominated his presidency. a former whig, lincoln ran on a political platform opposed to the expansion of slavery in the territories. his election served as the immediate impetus for the outbreak of the american civil war.