during the 1980s, changes in the air national guard's force structure and readiness were primarily driven by president reagan's military buildup and the need to prepare for a possible war between the north atlantic treaty organization (nato) and the warsaw pact in central europe. the ang focused on modernization, more realistic combat training, increased readiness, and personnel growth, primarily in nonflying, mission support units. in 1979, tactical air command assumed the atmospheric air defense mission of the united states with the inactivation of aerospace defense command (adcom or adc). adc fighter interceptor units were initially realigned into a component called air defense, tactical air command (adtac), at the level of an air division. in 1985, first air force (1 af) was reactivated by tac and given the mission to provide, train and equip adtac combat ready forces. upon its reactivation, first air force was composed of units of both the active air force and the air national guard. in the years since its third activation, more of the responsibility for the defense of american air sovereignty was shifted to the air national guard. by the 1990s, 90 percent of the air defense mission was being handled by the air national guard. in october 1997, first air force became an air national guard numbered air force, charged with the air defense of the north american continent.