WWII is closer to us than WWI and it involved some radically new techniques of warfare, including massive genocide, blitzkrieg, and atomic warfare. But WWI is really more important because it occurred thanks to the total failure of a century of European alliances which seemed to everyone to guarantee limited warfare and rational decision-making.
The uniting of various countries to counter aggressive threats, whether from old powers like France or new powers like Imperial Germany, was thought to be a surefire way of keeping the powder in Europe dry. It did not work for many reasons, including the rise of radical integral nationalism, the passion for new nations to satisfy ethnic groups which had never been organized in a common state structure, the intensified class struggles between labor and capital, the refinement of destructive weaponry, and the continuing attraction of imperialism, if not abroad then at home in Europe itself. As a result, events that in the past would not have started major war, like the assassination of an unimportant Austrian archduke when there were plenty of others to take his place, set off a campaign of imperialist and nationalist resentments that demonstrated that all the pacifistic talk of socialism and religion was meaningless. The war, expected to be short, turned out to be very long-lasting and many empires, hardly examples of virtuous government, fell to new-style states which would turn out to be so much worse that one could well long for the return of the Hapsburgs, the Hohenzollerns, the Ottomans, and even the unsavory Romanoffs. The war never really ended and was resumed in 1939. We probably should stop using the terms WWI and WWII and simply say "the World War of the 20th century."
After 1945 or so we started a new policy of alliances, NATO for example, to contain the Soviet Union, which had come to power as a result of WWI and had miraculously defeated Hitler almost single-handedly. We little tried to accommodate this new power in the world and relied on old methods. They seemed to have nevertheless worked, although so many little wars (Korean, Vietnam, etc.), when added up along with the rise of Red China, could well have amounted to a new catastrophe. We were lucky.
Are we going to go the same route now? Trump says no and many think that is a good thing, but he is no Bismarck, no Disraeli, no Churchill, and no FDR. He really doesn't know what to replace the alliance system with, and that could be the most deadly gift he offers the future.