The Banana Wars. The U.S. military interventions and occupations in Central America and the Caribbean during the 1920s and 1930s were not formally declared wars by Congress.
Presidents from Taft through Hoover sent Marines and naval forces into Nicaragua (occupied 1912–1925 and again 1926–1933), Haiti (1915–1934), the Dominican Republic (1916–1924), and shorter landings in Honduras, Guatemala, and Panama. Official justifications included protecting American lives and property, collecting debts, blocking European influence under the Roosevelt Corollary to the Monroe Doctrine, and restoring order after local unrest. Congress appropriated funds and received reports but never passed a declaration of war.
Read Smedley Butler’s book, War Is a Racket.