As simple as it seems, it is exactly the same reason children fight over their toys. They each believe that they have the right over the other person, they are willing to fight about it. This is true of brothers or cousins, or political countries.

It's almost always economic. You have something I want, so I decide to just TAKE it. Yes, there are other excuses. But the victor almost invariably comes away with more than they started with.

Territory, economics and raw materials (the Pheonecian wars, the Macedonian wars, the wars of ancient Greece, the wars of ancient Rome, the wars of the Khans, US vs Iraq II, Germany vs the world in 1914, the colonial wars, the wars between the Dutch and English, The Spanish and the English, The American Colonies and the English, Just about everybody and the English, Spain protecting her New World gold for the Genovese Banks, the US vs the Native Americans, US vs Spain, Soviet Union vs Afghanistan, Italy vs Ethiopia, Japan vs Russia, Japan vs Mongolia & China, Soviet Union vs China, European Countries vs China & the people of the New World & the people of Africa). Religion (the Crusades, the Muslim push into the Caucasus, the wars of the Reformation) Political Megalomania, racism, ideology (the Nazis against the world). Weapons/military efficiency testing (US vs Libya, US vs Granada, US vs Iraq I, Germany's involvement in the Spanish Civil War, covert US involvement in wars around the world, covert Soviet Union involvement around the world). Water Rights (San Salvador vs Nicaragua). A goading insult (the Franco Prussian War). Any excuse will do if you have a certain amount of your economy wrapped up in arms production.

Every war since the beginning of time was about religion or money or both.

War is sometimes theft. If a person can't get from you what he wants through honest means - he might try to take it from you. War is sometimes intolerance. Lets say you worship a different god than your neighbors in the next country, they might grow so intolerant of your beliefs as to invade you and force you to convert - or simply try to kill all of you. War is sometimes a smoke screen. A government in fear of loosing power, ( or just looking incompetent ), may drum beat up a conflict and start a war to preoccupy and distract the citizens of that land. War is sometimes economic. Sometimes a war can boost the economy when it is doing poorly - this happened in the United States During World War 2. War is sometimes a mistake. Nations have blundered into wars over mistakes - a misunderstood message, an accidental incursion, or a over zealous officer can start a conflict that may last many years and kill thousands. A few years ago the disputed results of a soccer match started a war - I kid you not. War is sometimes due to madness. Nations sometimes elect leaders who are - quite frankly mad. Hitler for example, Germany had no need or reason to invade Poland - but it was Hitlers "vision" of empire which was the cause of it. Napoleon of France had the same sort of "vision", and other examples exist all throughout history. War is sometimes "for fun". Don't laugh, I'm serious here - wars have been started out of boredom....usually in tribal and island cultures...literally they fight to the death because they have nothing better to do. There may be more reasons why men make war on their fellow men - insults have started wars, stray cows have started wars, bad treaties have started wars, even a runaway wife started a war or two - the variations are endless as to how wars start, but the hard reality of all of it is this - some people just seem to like wars - perhaps most of us do, we're a warlike people...and we can't seem to refrain from periodically - in a ritual manner - start killing our fellow humans for some trivial reason or other. I hope you find my answer useful to you.

for land and power fuelled by religious differences

because they are stupid enough to believe that fighting will help in any situation.

"People" is a little too imprecise and with too many answers. Individuals may have all sorts of motives for trying to start a war. Governments, however, start a war for one of two reasons - one criminal, the other defensive though pre-emptive. The first is they want what some other nation has, and may even believe or convince themselves they have right to it, and the right to take it by force. The second is that they fear that some current developments if left unchecked could leave them unable to adequately defend their sovereignty against other nations that would try to dominate or subjugate them in the future. An example of the first case is Germany's invasion of Poland in 1939 -- and every other time Germans invaded Poland. An example of the second is Britain's frequent wars with major continental powers which took control of Belgium - "The Pistol aimed at Britain's heart!" It's why they declared war on Germany in World War I, and why they declared War on the French Republic in the 1790s, why they declared war on France under Louis XIV, and why they kept fighting the Spanish in the 1500s. Their future security and sovereignty required that Belgium be indpependent or in the hands of a weak overlord that posed no threat to England. Virtually all of Europe played the latter game - the Balance of Power - for most of European History: any time one nation was getting so powerful as to be approaching a position where they could subjugate all Europe, the other powers banded together to stop them. It is important to note, that in many of these cases the power threatening domination might not have done anything wrong to get there: Charles V legally inherited the throne of Spain and its American Empire, plus Italy, Germany, Hungary, Austria, Belgium, the Netherlands, Bohemia (the Czech Republic today), western Poland, eastern France, and the Rhineland - but that didn't matter to the rest of Europe: it was break-up the Spanish Empire then or be subjugated by it later.

Every conflict begins from within.