Declaration of war
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Declarations of war have been acceptable means and diplomatic measures since the Renaissance, when the first formal declarations of war were issued.
In public international law, a declaration of war entails the recognition between countries of a state of hostilities between these countries, and such declaration acted to regulate the conduct between the military engagements between the forces of the respective countries. The primary multilateral treaties governing such declarations are the Hague Conventions.
The League of Nations formed in 1919 in the wake of the First World War, and the General Treaty for the Renunciation of War of 1928 signed in Paris, demonstrated that world powers were seriously seeking a means to prevent the carnage of the world war. However, these powers were unable to stop the Second World War and, thus, the United Nations System was put in place after that war in an attempt to prevent international aggression through declarations of war.
 The UN and war
In an effort to force nations to resolve issues without warfare, framers of the United Nations Charter attempted to commit member nations to using warfare only under limited circumstances, especially for defensive purposes only.
The UN paradoxically became a war combatant itself after North Korea invaded South Korea on 15 June 1950. The United Nations Security Council condemned the North Korean action by a 9-0 resolution (with the Soviet Union absent) and called upon its member nations to come to the aid of South Korea. The United States of America and 15 other nations formed a "UN force" to pursue this action. In a press conference on 29 June 1950, U.S. President Truman characterized these hostilities as not being a "war," but a "police action."
 Undeclared wars
In most democratic nations, a Declaration of War customarily has to be passed by the legislature. In the United States there is no format required for declaration(s) of war. The term "Declaration of War" is not, in fact, mentioned by the US Constitution. Instead the Constitution says "Congress shall have the power to ... declare War, ..." without defining the form such declarations will take. Therefore, many have argued congressionally passed authorizations to use military force are "Declarations of War." That concept has never been tested in the US Court system. Some, such as Congressman Ron Paul, argue that an explicit Declaration of War is, in fact, a Constitutional requirement. <ref name="ron paul">http://www.house.gov/paul/press/press2002/pr100402.htm</ref>
After the United Nations action in Korea, a number of democratic governments pursued usually limited warfare by characterizing them as something else, such as a "military action" or "armed response." This was most notably used by the United States in its more than decade-long involvement in Vietnam. Nations such as France, which had extensive colonies in which its military provided order, continued to intervene in their former colonies' affairs as police actions since they could no longer be deemed internal conflicts.
Not declaring war provides a way to circumvent constitutional safeguards against the executive declaring war, and also, in some cases, to avoid feeling bound by the established laws of war. Not using the word "war" is also seen as being more public relations-friendly. For these reasons, they have generally ceased to issue declarations of war, instead describing their actions by euphemisms such as "police action" or "authorized use of force."
 Authorized use of force
Frequently used as an alternative to a declaration of war, authorized use of force is often used to avoid traditional barriers to the initiation of combat. Typically a full declaration must be ratified by various legislative bodies, but 'authorized use of force' may allow an elected head of state to directly initiate forceful action without further consultation. In addition, with declarations of war being increasingly regulated by international bodies, 'authorized use of force' can often be used to avoid some of the negative consequences of a declaration.
Authorized use of force is relatively common among democratic societies. The United States, for instance, has been directly involved in military activities in every decade of the latter half of the twentieth century yet has not declared war formally since World War II. For instance, in the case of the Vietnam war and the Iraq war, Congress authorized the use of force rather than putting forth a declaration of war. As noted above, there is a dispute over the constitutionality of this legislative procedure.
 Declarations of war during World War II
- September 3
- Great Britain, France, Australia and New Zealand declare war on Germany.
- September 6
- The Union of South Africa declares war on Germany.
- September 10
- Canada declares war on Germany.
- April 9
- Norway declares war on Germany.
- May 10
- The Netherlands declare war on Germany.
- June 10
- Italy declares war on France and Great Britain.
- June 11
- France declares war on Italy. Great Britain, Australia, Canada, New Zealand and the Union of South Africa at war with Italy.
- October 28
- Italy declares war on Greece.
- November 23
- Belgium declares war on Italy.
- April 6
- Italy declares war on Yugoslavia.
- April 24
- Bulgaria declares war on Greece and Yugoslavia.
- June 22
- Germany, Italy, and Romania declare war on the Soviet Union.
- June 25
- Finland officially notes that a state of war exists with the Soviet Union (no parliamentary motion was passed).
- June 27
- Hungary declares war on the Soviet Union.
- December 6
- Great Britain declares war on Finland and Romania.
- December 7
- Japan declares war on the US, Great Britain, Australia, Canada, New Zealand and the Union of South Africa. Great Britain, Australia and New Zealand declare war on Finland, Hungary and Romania. Canada declares war on Finland, Hungary, Japan and Romania. Panama declares war on Japan. Yugoslavia at war with Japan.
- December 8
- US, Great Britain, Australia, Costa Rica, The Dominican Republic, El Salvador, Haiti, Honduras, The Netherlands, New Zealand and Nicaragua declare war on Japan
- December 11
- Germany and Italy declare war on the United States of America.
- December 12
- Bulgaria declares war on the United States of America and Great Britain. Romania declares war on the United States of America.
- December 13
- Hungary declares war on the United States of America.
 Current declarations
As of 2005, a few declarations of war remain in effect, though they are usually retained for lack of a peace treaty rather than reflecting an active state of hostilities.
- North and South Korea have remained legally at war since the Korean War.
- Israel has been at war with Syria since the Yom Kippur War.
- Having refused to sign the 1949 Armistice Agreements, Iraq has remained in a state of war with Israel ever since.
- Armenia and Azerbaijan remain deadlocked on the issue of Nagorno-Karabakh and have yet to find a peaceful resolution.
- Russia and Japan technically never ended their state of war following World War II. The Kuril Islands dispute is the current obstacle to the signing of a peace treaty.
- The Netherlands has never signed a peace treaty with Portugal after a conflict in 1567; thus, the two are technically still at war with each other. 
 See also
- Ongoing wars (mostly undeclared)
- Declaration of war by the United States
- German declaration of war against the Netherlands (May 10 1940)