Giving the reasons for war covering several centuries in our world is a daunting and unrealistic task. I would like to, instead, focus on the United States in its relatively short history. And instead of detailing the many reasons for American war, I would like to simply point out if these wars were necessary (N) or unnecessary (U).
I will not be rating relatively minor wars such as the Indian Wars (1817-1898) where the U.S. attacked Native Americans, The Banana Wars (1907-1934) where the U.S. occupied countries in Central America and the Caribbean, NATO wars such as in Yugoslavia, and U.N. wars such as in Somalia.
(N) The American Revotutionary War (1775-1783) - This was an obviously necessary war to free the American people from the grips of British oppression.
(U) The War of 1812 (1812-1815) - The United States was the first to declare war in its expansionist effort to seize Upper Canada (now Ontario) while Britain was busy with the much-larger Napoleonic wars raging in Europe. Former president and a Founding Father Thomas Jefferson said in 1812, "The acquisition of Canada this year, as far as the neighborhood of Quebec, will be a mere matter of marching, and will give us the experience for the attack on Halifax, the next and final expulsion of England from the American continent."
(U) The Mexican-American War (1846-1848) - This war was another expansionist effort by the United States to gain territory. At the completion of this war, the U.S. obtained 55% of Mexican national territory. At the time, U.S. Congressman Abraham Lincoln spoke out against the war. President Ulysses S Grant, who was a young army officer in this war, recalled in his 1885 memoirs, "For myself, I was bitterly opposed to the measure (Mexican War), and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger nation against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory."
(?) The American Civil War (1861-1865) - This war could have gone either way. It did save the United States from falling apart, but it also took away the self-determination of the people of several states. This war was the deadliest war in U.S. history, killing 620,000 soldiers and an undetermined number of civilians.
(U) The Spanish-American War (1898) - This was a war that the United States did not have to start. Sure, we have the sinking of the USS Maine in Havana harbor under dubious circumstances and the desire to help Cubans free themselves from Spain. But in the end, the U.S. gained more territory - Cuba, the Philippines, Puerto Rico and Guam. The later two still are U.S. territories.
(U) The First World War (1917-1918 U.S. involvement) - This was strictly an European war, although Britain did use soldiers from her empire such as Canada. Without U.S. involvement, however, this war would have simply ended in a stalemate, maybe an exchange of some land, and a peace treaty.
(N) The second World War (1942-1945 U.S. involvement) - Without U.S. involvement, Japan and Germany would certainly have conquered all of their targets. Britain may have been spared, but she would have remained isolated. Meanwhile. the U.S. would have been alone in an Axis-controlled world with no allies. The United States had to go to war in this case to defeat the very real threat of Nazi Germany and Imperial Japan.
(N) The Korean War (1950-1953) - When one country invades another, war is inevitable. The United States, under the authority of the U.N., pushed out the North Korean invaders from South Korea.
(U) The Vietnam War (1962-1973) - The United States, under three presidents, lost over 50,000 soldiers. A generation of young men who died for absolutely nothing.
(N) The Persian Gulf War (1991) - Another case of an invasion. The United States and several other countries were able to expel the Iraqi invaders from Kuwait.
(N) The Afghanistan War (2001-present) - Afghanistan is the country in which 911 was planned. It is the country in which the terrorist of 911 trained. This war is the real war on terror. The United States and several other countries invaded Afghanistan to stamp out terrorism and must stay there until the war is won.
(U) The Iraq War (2003-present) - What a colossal mistake was this war. It ruined Iraq which was in stable condition under Saddam. There were no terrorists in Iraq under Saddam. Now it is a training ground for terrorists. Iraq under Saddam was a country that Iran feared. Now Iran fuels terrorism in Iraq. What a different world we would have had if not for this war.
There you have it. In my opinion, the United States in its short history, has participated in five necessary major wars and an astonishing six unnecessary major wars. Two of those unnecessary wars were costly in the terms of lives and money. Another three of those unnecessary wars were nothing but land grabs.