CREATE TABLE table_78291 ("Disaster" TEXT, "Year" TEXT, "Countries Affected" TEXT, "Scale of Disaster" TEXT, "People Assisted" TEXT, "Nature of Help" TEXT)

Which year did USA undergo a disaster?
SELECT "Year" FROM table_78291 WHERE "Countries Affected" = 'usa'
SELECT "Year" FROM "table_78291" WHERE "Countries Affected" = 'usa'