> WWII started just before WWI. Some people call the Second World War part 2 of the first.
Ehh......
WWI is complicated, but it essentially boils down to a great power war caused by a breakdown in relations that reached the point that diplomats were unwilling or incapable of keeping the war from breaking out. The result of WWI was that all of the traditional great powers (both those who won and lost) were spent [1]. The peace treaty sought to see the victors compensated by the losers, and part of the compensation was breaking them up in the vain hope that this would make war less likely, with some parts being carved up into independent countries, and others (especially colonies) being annexed to the victors.
WWII isn't so much a single war as it is four (sets of) wars of naked territorial aggression (Germany, Italy, Soviet, and Japanese) and two civil wars (China and France) that got merged into a single conflict by the fact that everyone ended up aligning into one side or the other. These wars don't start just before WWI; in many cases, the territorial jealousies that precipitate the war can't start until after WWI (e.g., how can Russia start seeking to invade its neighboring countries when they're still part of Russia?).
In between these two conflicts is a very large series of civil wars and revolutions and failed revolutions that are largely born from the instability of the international political sphere following the exhaustion of all great powers in WWI. These (relatively) smaller conflicts provide a more or less continuous segue between WWI and WWII, to the point that it may be better to just think of the period from 1914 to 1949 as a modern Thirty Years' War that sees the world shift from a balance-of-power regime involving the major European powers to a world that involves just two superpowers and their alliances.
[1] The US was the only major country not economically devastated by the war, but despite its economic size, its unwillingness to participate in European affairs means it's not really a great power as far as people at the time were heavily concerned--it doesn't enter the stage until WWI.