The United States, which secretly supported Gen. Manuel A. Noriega for years, has been seeking to dislodge the Panamanian strongman since 1987. Here is a chronology of major events in Panama's recent history: April, 1978--U.S. Senate ratifies Panama Canal Treaties, which will turn over canal and U.S. military bases to Panamanian control on Dec. 31, 1999. Until then, U.S. troops are to remain in Panama and provide security for canal. July, 1981--Panamanian dictator Omar Torrijos, who seized power in 1968 coup, dies in mysterious plane crash. His intelligence chief, Col. Manuel A. Noriega, gradually assumes control of armed forces and emerges as de facto leader of nation. It is later learned that Noriega was a paid informer for CIA and, perhaps, other nations' intelligence services. May, 1984--Nicolas Ardito Barletta elected president on military-backed slate chosen by Noriega. Opposition leaders allege fraud in voting. September, 1985--Noriega removes Ardito Barletta from power and installs Eric A. Delvalle. June, 1987--Col. Roberto Diaz Herrera, ousted chief of staff of Panama Defense Forces, accuses Noriega of election fraud, drug trafficking and involvement in deaths of Torrijos and other political rivals. National Civic Crusade--coalition of 101 professional, labor and business groups--formed and issues call for Noriega's resignation. July, 1987--Police close independent newspapers and raid headquarters of Civic Crusade to arrest five leaders. They are later freed and flee country. December, 1987--U.S. Senate bans military and economic aid to Panama. Feb. 5, 1988--Federal grand juries in Tampa and Miami indict Noriega on drug-trafficking charges. Feb. 25, 1988--President Delvalle announces he will dismiss Noriega as defense chief and replace him with Col. Marcos Justines, second-in-command. Justines refuses to take job. Feb. 26, 1988--National Assembly, under orders from Noriega, ousts Delvalle and names Education Minister Manuel Solis Palma new president. Delvalle goes into hiding. March, 1988--Opposition groups proclaim Delvalle president of a "government of national reconciliation." President Ronald Reagan imposes economic sanctions against "illegitimate" Noriega regime and freezes $7 million in canal revenue and Panamanian assets held by U.S. banks. March 16, 1988--Troops loyal to Noriega crush coup attempt led by Panama City Police Chief Col. Leonidas Macias even as street protests and power blackouts bring city to grinding halt. April, 1988--United States dispatches 1,300 troops to Panama to bolster security for U.S. armed forces and their dependents. Noriega calls move a prelude to U.S.-inspired coup. May 25-26, 1988--Secretary of State George P. Shultz, accusing Noriega of backing out of deal at last moment, announces that talks on his departure from power have collapsed and no further contacts are contemplated. Noriega condemns American meddling in Panamanian affairs. September, 1988--Forces loyal to Noriega arrest 26 Panamanians for allegedly conspiring with U.S. government to mount an effort to topple dictator. May 7-8, 1989--Panama conducts national elections. Foreign observers claim opposition candidates, led by Guillermo Endara, win by large margin. But Noriega government annuls elections, charging foreign interference. May 10, 1989--Roving pro-Noriega street fighters called Dignity Battalions attack opposition candidates who are marching in protest. Vice presidential candidate Guillermo Ford severely beaten as police stand by. Ford's bodyguard killed. Sept. 1, 1989--After months of mediation efforts by Organization of American States fail to resolve crisis, Noriega names unknown bureaucrat Francisco Rodriguez as new president. Oct. 3, 1989--Rebel troops and junior officers stage coup that nearly succeeds in toppling Noriega. U.S. troops provide only half-hearted help as U.S. officials acknowledge they knew of attempt but decided to remain at arm's length from coup. Dec. 15, 1989--Puppet assembly grants Noriega wider government powers and declares that Panama is in state of war with United States. Dec. 16, 1989--Unarmed U.S. military officer in civilian clothes shot and killed by Panama Defense Forces troops near Noriega's headquarters. Dec. 20, 1989--United States launches a military assault on Panama with the declared aims of restoring democracy and capturing Noriega.