There were many things that led to WWI. The world was changing; old empires were losing control of colonies, various ethnic groups in different empires wanted independence; the causes were many, and complicated. The assasination of Archduke Ferdinand and his wife was not a "cause", but it was the spark that lit the fuse of an unstable bomb...
Power-hungry individuals seem to be the cause of most wars in history as they want to constantly expand their boundaries and conquer more territory. I do agree that the States seemed to win quite a lot for a side that only joined quite late on in the game, but crucially it was the support of the States that won the war for the allies, in my opinion.
I would agree with previous posts that greed was a major cause of World War II. In Europe Hitler and his desire to take over Europe forced everyone else into the war out of fear.
The biggest beneficiaries of these two wars together was the United States, hands down, as it assured the US status as a world power, then a superpower, and limited imperial competition to basically just the Soviet threat.
I would say Japan benefited a lot from World War I - it came out on the winning (Allied) side, was allowed to construct more of a navy and a modern military after the war, and saw other empires (Russia and China) thrown into chaos.
World War 1 and The Depression were causes of World War II. The winners of World War II include many countries: Britain, United States, France, China,the Soviet Union, Canada, etc... The countries that were allies to the above countries also "won" WWII.
WWI was caused by the assassination of the Archduke Ferdinand of Austria on June 28, 1914. He and his wife, Sophie, were killed in Sarajevo.
WWII, at least for the US, was begun with the bombing of Pearl Harbor on December 7, 1941. It began much earlier than that in Europe, of course, with the Hitler regime attempting to take over much of Europe and spreading the anti-semitic propaganda as it went.
I'll add greed as another major cause of World War II. Hitler's constant desire to expand Germany's borders forced England and France to fight back just to preserve their own sovereignty. Certainly no two nations benefitted more greatly than the U. S. and the Soviet Union. The U. S. cemented their claim as a superpower, while the Soviet Union annexed many former Eastern European nations.
The other winner in WWII was the Soviet Union. They suddenly had the continent of Europe more or less to themselves -- no credible threat.
I'll go with fear as a cause of the wars. Most countries feared what would happen if rivals got stronger. This made them try to make themselves stronger, leading to arms races and such. (I disagree with the idea that building a military makes you more likely to use it, though.) So, for example, Japan feared that the US would try to cut it off from gaining power so it tried to expand its empire. The US saw that as a threat to it -- both countries had been worrying about the other for decades...
So I'd go with fear of the intentions of others as the main cause of the wars.
In terms of benefit, you have to look at the United States as one of the biggest winners. Not only was our country not completely devestated as much of Europe and Japan were, we also had the fewest casualties of any of the major players, the fewest civilian casualties, and the massive demand for equipment and ammunition helped to an enormous extent in leading the country out of the depression.
Of course it is easy for me to write all of that because I didn't lose anyone in the war or have to fight and kill for my country. But if we are simplifying things, I think it would be very difficult to point to a bigger winner or a country that benefitted more!
In terms of the causes, WWI was certainly a big cause of WWII but greed, ambition, lust for power, all of these come to mind as pretty good causes. That and the still lingering idea that war is somehow glorious and that a country can actually win or lose a war. All of those helped. And once the European nations started building up all those armies and navies, they had to use them didn't they?