Declaration of war by the United States A declaration of war is a formal declaration issued by a national government indicating that a state of war exists between that nation and another . Overview . For the United States , says " Congress shall have power to . . . declare War " . However , that passage provides no specific format for what form legislation must have in order to be considered a " Declaration of War " nor does the Constitution itself use this term . Many have postulated " Declaration ( s ) of War " must contain that phrase as or within the title . Others oppose that reasoning . In the courts , the United States First Circuit Court of Appeals in " Doe v . Bush " said : " [ T ] he text of the October Resolution itself spells out justifications for a war and frames itself as an ' authorization ' of such a war . " in effect saying an authorization suffices for declaration and what some may view as a formal Congressional " Declaration of War " was not required by the Constitution . This article will use the term " formal Declaration of War " to mean Congressional legislation that uses the phrase " Declaration of War " in the title . Elsewhere , this article will use the terms " authorized by Congress " , " funded by Congress " or " undeclared war " to describe other such conflicts . The United States has formally declared war against foreign nations five separate times , each upon prior request by the President of the United States . Four of those five declarations came after hostilities had begun . James Madison reported that in the Federal Convention of 1787 , the phrase " make war " was changed to " declare war " in order to leave to the Executive the power to repel sudden attacks but not to commence war without the explicit approval of Congress . Debate continues as to the legal extent of the President ' s authority in this regard . After Congress repealed the Gulf of Tonkin Resolution in January 1971 and President Richard Nixon continued to wage war in Vietnam , Congress passed the War Powers Resolution ( ) over the veto of Nixon in an attempt to rein in some of the president ' s claimed powers . The War Powers Resolution proscribes the only power of the president to wage war which is recognized by Congress . Declarations of war . Formal . The table below lists the five wars in which the United States has formally declared war against eleven foreign nations . The only country against which the United States has declared war more than once is Germany , against which the United States has declared war twice ( though a case could be made for Hungary as a successor state to Austria - Hungary ) . In World War II , the Japanese attacked Pearl Harbor on December 7 , 1941 . Germany and Italy , led respectively by Adolf Hitler and Benito Mussolini , declared war on the United States , and the U . S . Congress responded in kind . Military engagements authorized by Congress . In other instances , the United States has engaged in extended military combat that was authorized by Congress . Military engagements authorized by United Nations Security Council Resolutions and funded by Congress . In many instances , the United States has engaged in extended military engagements that were authorized by United Nations Security Council Resolutions and funded by appropriations from Congress . Other undeclared wars . On at least 125 occasions , the President has acted without prior express military authorization from Congress . These include instances in which the United States fought in the Philippine - American War from 1898 – 1903 , in Nicaragua in 1927 , as well as the NATO bombing campaign of Yugoslavia in 1999 . The United States ' longest war was fought between approximately 1840 and 1886 against the Apache Nation . During that entire 46 - year period , there was never more than 90 days of peace . The Indian Wars comprise at least 28 conflicts and engagements . These localized conflicts , with Native Americans , began with European colonists coming to North America , long before the establishment of the United States . For the purpose of this discussion , the Indian Wars are defined as conflicts with the United States of America . They begin as one front in the American Revolutionary War in 1775 and had concluded by 1918 . The United States Army still maintains a campaign streamer for Pine Ridge 1890 – 1891 despite opposition from certain Native American groups . The American Civil War was not an international conflict under the laws of war , because the Confederate States of America was not a government that had been granted full diplomatic recognition as a sovereign nation by other sovereign states . The CSA was recognized by the United States government as a belligerent power , a different status of recognition that authorized Confederate warships to visit non - U . S . ports . This recognition of the CSA ' s status as a belligerent power did not impose any duty upon the United States to recognize the sovereignty of the Confederacy , and the United States never did so . The War Powers Resolution . In 1973 , following the withdrawal of most American troops from the Vietnam War , a debate emerged about the extent of presidential power in deploying troops without a declaration of war . A compromise in the debate was reached with the War Powers Resolution . This act clearly defined how many soldiers could be deployed by the President of the United States and for how long . It also required formal reports by the President to Congress regarding the status of such deployments , and limited the total amount of time that American forces could be deployed without a formal declaration of war . Although the constitutionality of the act has never been tested , it is usually followed , most notably during the Grenada Conflict , the Panamanian Conflict , the Somalia Conflict , the Gulf War , and the Iraq War . The only exception was President Clinton ' s use of U . S . troops in the 78 - day NATO air campaign against Yugoslavia during the Kosovo War . In all other cases , the President asserted the constitutional authority to commit troops without the necessity of Congressional approval , but in each case the President received Congressional authorization that satisfied the provisions of the War Powers Act . On March 21 , 2011 , a number of lawmakers expressed concern that the decision of President Barack Obama to order the U . S . military to join in attacks of Libyan air defenses and government forces exceeded his constitutional authority because the decision was made to authorize the attack without Congressional permission .