U.S. Constitution - Article 1 Section 8.
Article 1 - The Legislative Branch Section 8 - Powers of Congress
To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water.
Revisionists are typically quick to condemn President Franklin Roosevelt for his actions, which cast the United States into the Second World War. While the media and public opinion voice virtually no doubt that World War Two was a moral war (for the Allies) and one that needed to be fought, revisionists have frequently analyzed Roosevelt's actions that broke his 1940 campaign promise to keep Americans out of any foreign war. One of the foremost figures of World War Two historical revisionism, Harry Elmer Barnes, wrote,
"[Roosevelt] went as far as he dared in illegal efforts, such as convoying vessels carrying munitions, to provoke Germany and Italy to make war on the United States. Failing in this, he turned to a successful attempt to enter the War through the back door of Japan. He rejected repeated and sincere Japanese proposals that even [Cordell] Hull admitted protected all the vital interests of the United States in the Far East, by his economic strangulation in the summer of 1941 forced the Japanese into an attack on Pearl Harbor, took steps to prevent the Pearl Harbor commanders, General Short and Admiral Kimmel, from having their own decoding facilities to detect a Japanese attack, kept Short and Kimmel from receiving the decoded Japanese intercepts that Washington picked up and indicated that war might come at any moment, and ordered General Marshall and Admiral Stark not to send any warning to Short and Kimmel before noon on December 7th, when Roosevelt knew that any warning sent would be too late to avert the Japanese attack at 1:00 P.M., Washington time"
Despite Roosevelt's maneuvering that resulted in war with Japan and five European nations including Germany, Roosevelt was the last U.S. President who didn't skirt the US Constitution and actually went to war only following a formal declaration by Congress.
President Franklin D. Roosevelt signing the declaration of war against Germany, marking US entry into World War Two in Europe. Senator Tom Connally stands by holding a watch to fix the exact time of the declaration. 11 December 1941. By Farm Security Administration/Office of War Information [Public domain], via Wikimedia Commons
The events surrounding the declaration of war on Japan are fairly well known. On December 8, 1941, the day after the Japanese attacked Pearl Harbor, Roosevelt delivered his famous "Day of Infamy" speech. The address concluded with his request that Congress formally declare war,
"I ask that the Congress declare that since the unprovoked and dastardly attack by Japan on Sunday, December 7, 1941, a state of war has existed between the United States and the Japanese Empire."
Immediately afterward, the Senate unanimously approved the resolution 82-0, while the House of Representatives vote was 388 to 1. The one dissenting vote was from Montana Republican Jeannette Rankin.
Three days later, following Hitler's declaration of war on the United States, Roosevelt again went to Congress, to request a recognized state of war with both Germany and Italy. This time the vote was unanimous (Rankin would vote "present" rather than for or against the declaration).
Roosevelt would request Congress to declare war once again on June 5, 1942. Three declarations of war were issued that day, against Bulgaria, Hungary, and Romania. The declaration of war followed a request from Roosevelt issued on June 2, 1942. In his message he wrote simply:
"The Governments of Bulgaria, Hungary, and Romania have declared war against the United States. I realize that the three Governments took this action not upon their own initiative or in response to the wishes of their own peoples but as the instruments of Hitler. These three Governments are now engaged in military activities directed against the United Nations and are planning an extension of these activities.
Therefore, I recommend that the Congress recognize a state of war between the United States and Bulgaria, between the United States and Hungary, and between the United States and Romania. "
The declaration of war against Bulgaria, Hungary and Romania stands today as the last such declaration by the United States.
In all, the United States has only made 11 separate formal declarations of war against foreign nations encompassing 5 wars. 
- War with Great Britain 1812 (Act of June 18, 1812, House 79-49; Senate 19-13)
- War with Mexico 1846 (Act of May 13, 1846, House 174-14; Senate 40-2)
- War with Spain 1898 (Act of April 25, 1898, House & Senate voice votes)
- War with Germany 1917 (Act of April 6, 1917, House 373-50; Senate 82-6)
- War with Austria-Hungary 1917 (Act of December 7, 1917, House 365-1; Senate 74-0))
- War with Japan 1941 (Act of December 8, 1941, House 388-1; Senate 82-0)
- War with Germany 1941 (Act of December 11, 1941, House 393-0; Senate 88-0)
- War with Italy 1941 (Act of December 11, 1941, House 399-0; Senate 90-0)
- War with Bulgaria 1942 (Act of June 5, 1942, House 357-0; Senate 73-0)
- War with Hungary 1942 (Act of June 5, 1942, House 360-0; Senate 73-0)
- War with Romania 1942 (Act of June 5, 1942, House 361-0; Senate 73-0)
Since 1942, of course the United States has not led the world in a seventy-years' peace. Despite Roosevelt's relative ease in obtaining six formal declarations of war, since World War Two Americans have been drawn time and time again into war without a congressional declaration. Gore Vidal commented:
"Since V-J Day 1945 ('Victory over Japan' and the end of World War II), we have been engaged in what the historian Charles A. Beard called 'perpetual war for perpetual peace.' I have occasionally referred to our 'enemy of the month club': each month we are confronted by a new horrendous enemy at whom we must strike before he destroys us."
Vidal goes on to list several hundred wars and operations conducted against Communism, terrorism, drugs, or as he puts it, "sometimes nothing much" that occurred between Pearl Harbor and September 11, 2001.  Based on casualties alone the costliest conflicts following the last official declaration of war include the Korean War with 33,686, the Vietnam War with 47,424, Iraq War with 3,542 and Afghanistan at greater than 2,000.
While generally remembered as one of the United States's costliest wars, the Korean War was referred to only as a "Police Action" by then-President Harry Truman. Truman announced on June 27, 1950 that he ordered U.S. air and naval forces to South Korea to aid their army in repulsing an invasion by Communist North Korea. Truman justified his actions by explaining that he was enforcing a United Nations resolution calling for an end to hostilities, and to stem the spread of Communism in Asia. 
Truman's actions set a precedent that would be followed by Democrats and Republicans to the present day. With the path now cleared of any potential congressional opposition, U.S. presidents would be empowered to conduct the wars of their choosing. The Vietnam War with over 47,424 dead also was fought without a declaration of war. President Lyndon Johnson would issue a report claiming two attacks against U.S. ships in the Gulf of Tonkin. The so-called "Gulf of Tonkin Resolution" passed in August 1964 gave President Johnson free rein to escalate the war. 
In March 2003, forces from the United States, the United Kingdom, Australia and Poland invaded Iraq. According to U.S. President George W. Bush and British Prime Minister Tony Blair, the coalition mission was "to disarm Iraq of weapons of mass destruction, to end Saddam Hussein's support for terrorism, and to free the Iraqi people." Like the "events" of the Gulf of Tonkin, the "Weapons of Mass Destruction" proved to be nonexistent. Whether Bush was simply mistaken or whether he fabricated fantastic weapons to garner support for his war, it is clear that such power should not be in the hands of the President alone.
The U.S. Constitution was purposely designed to prevent such power to reside with the president. James Wilson, a Constitutional Convention delegate, explained to the Pennsylvania ratifying convention in 1787,
"This system will not hurry us into war; it is calculated to guard against it. It will not be in the power of a single man, or a single body of men, to involve us in such distress; for the important power in declaring war is invested in the legislature at large." 
How then did such absolute power shift to the executive branch? Ron Paul attempts to answer:
"Congress has either ignored its responsibility entirely over these years, or transferred the war power to the executive branch by a near majority vote of its members, without consideration of it by the states as an amendment required by the Constitution."
Today, Americans continue to fight and die all around the world. The aggression of the United States would be universally condemned if launched by any other nation.
We are left to wonder, if the matter were left to the American people and their representatives in Congress, how many lives would not have been lost over these past 70 years? How many dollars would not have been wasted?
In the 1940 presidential election campaign, Roosevelt promised to keep America out of the war. He stated, "I have said this before, but I shall say it again and again and again; your boys are not going to be sent into any foreign wars." His position was popular and led to his unprecedented third term as president. In a recent poll 60% of Americans said the war in Afghanistan is not worth fighting. Despite a 70-year media war against "isolationism" the American people still favor peace and keeping out of foreign conflicts despite the perpetual series of wars launched by our presidents. The time has come for revisionists to consider the actions of all those who have followed Roosevelt. For all his lies and maneuvering, Roosevelt looks like a great statesman compared to those who have followed right down to and including the current commander-in-chief.
|||Some of the most important examples are Charles Callan Tansill’s Back Door to War: The Roosevelt Foreign Policy 1933-1941, Charles Beard’s President Roosevelt and the Coming of the War 1941, Thomas Fleming’s The New Dealer’s War: F.D.R. and the War within World War II.|
|||Harry Elmer Barnes, “Revisionism and the Promotion of Peace” in Barnes against the Blackout, (Costa Mesa: Institute for Historical Review, 1991), pp. 285-86.|
|||After being booed and hissed by other members of Congress, Jeannette Rankin explained her position, “As a woman, I can't go to war and I refuse to send anyone else.” Rankin was later vilified in the press and dubbed, “Japanette Rankin.” Rankin never apologized for her vote. Through the years she continued to be an advocate for pacifism and even led a campaign against US involvement in Vietnam in 1968 when she was 87 years old. See “Jeannette Rankin casts sole vote against WWII” online: http://www.history.com/this-day-in-history/jeanette-rankin-casts-sole-vote-against-wwii and “The First Woman in Congress: A Crusader for Peace” online: http://www.npr.org/2011/07/14/135521203/the-first-woman-in-congress-a-crusader-for-peace|
|||For the text of Hitler’s Declaration of War against the United States see The Journal of Historical Review, Vol. 8, No. 4, Winter 1988-89, pp.389-416, online: http://www.ihr.org/jhr/v08/v08p389_Hitler.html|
|||Congressional Record, June 2, 1942, H Doc. No. 761, pg. 4946. Online: http://www.ibiblio.org/pha/policy/1942/420602a.html|
|||Jennifer K. Elsea and Richard F. Grimmett, “Declarations of War and Authorizations for the Use of Military Force: Historical Background and Legal Implications,” Congressional Research Service, March 17, 2011. Online: http://www.fas.org/sgp/crs/natsec/RL31133.pdf|
|||Gore Vidal, Perpetual War for Perpetual Peace: How We Got to Be So Hated (New York: Nation Books, 2002), pp. 20-21.|
|||Ibid. p. 40.|
|||United States military casualties of war online: http://en.wikipedia.org/wiki/United_States_military_casualties_of_war While this article cites 1,432 deaths in Afghanistan, the actual number is higher and still growing. See for example: “US Casualties in Afghanistan Rise Past 2,000 as Long-Term Costs of War Remain Unknown,” online: http://www.huffingtonpost.com/2012/09/30/casualties-afghanistan-war_n_1927691.html|
|||Online: http://www.history.com/this-day-in-history/truman-orders-us-forces-to-korea See also: http://www.nytimes.com/learning/general/onthisday/big/0627.html|
|||Due to the declassification of National Security Agency documents, it is now known that the second attack on U.S. ships in the Gulf did not occur. See: http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB132/press20051201.htm|
|||See: “CIA’s final report: No WMD found in Iraq” Online: http://www.msnbc.msn.com/id/7634313/ns/world_news-mideast_n_africa/t/cias-final-report-no-wmd-found-iraq/#.UOCedY7bwyE|
|||Cato Handbook for Policymakers, CATO Institute 7th Edition. “10: Reclaiming the War Power.” Online: http://www.cato.org/sites/cato.org/files/serials/files/cato-handbook-policymakers/2009/9/hb111-10.pdf|
|||Ron Paul, “Violating the Constitution with an Illegal War.” Ron Paul’s otherwise excellent article mistakenly asserts that the last time Congress declared war was against Germany on December 11, 1941. Online: http://www.lewrockwell.com/paul/paul57.html|