Let us take each war separately. First, WWI, then WWII
WWI:
World War I was almost inevitable. The tensions between Serbia and Austrio-Hungary grew beyond control, with little chance of being fixed. Also, as war was inevitable, it led to countries creating large and complicated military alliances, which soon led to the "World" war.
Causes:
The fuse for the war finally reached detonation, when:
1) Austrio-Hungary annexed territories that Serbia wanted to annex, leading to increased tension
2) Serbian Princip assasinated Archduke Franz Ferdinand and his wife Sophie, triggering the war.
3) Austrio-Hungary did not accept the compensation at first put up by Serbia, and, to be able to take over all of Serbia, declared war in hope of annexing Serbia.
This led to each country declaring war successively because of their pledge to help each other, leading it to a global war
Effects of the War:
After 4 bloody years of war, and large technological advancements (in compared to the previous wars), death and wounded tolls were drastic, and even went up to the millions. There were large amounts of effects, in which some are connected with the start of WWII. To start, let us list some changes that did not have a large effect on WWII:
1) Dissolvement of the Ottoman Empire and Austrio-Hungary: As part of the treaty, the two countries were dissolved, and were broken to smaller states. Austrio-Hungary became two nations, Austria and Hungary, while the Ottoman Empire split into several smaller states that are located in the Middle East, with the largest chunk being present-day Turkey.
Some causes that led to the start of WWII:
1) War reparation: Seen as unfair in today's society, the Allies decided to put most of the war costs on Germany (even though they did not actually want the war to start, and even warned Austrio-Hungary against it.)
2) Great Depression: Because of the above, the Kaiser became unpopular, and had to step down. Large amounts of debt meant more bank-note printing, which led to the decrease in value of German currency.
3) Rise of Nazi Party: Combined with a downturn of the economy, and then the Great Depression on top of that, the then small Nazi Party was able to grow in size, and was able to coup de'tat the government, replacing the conventional government with Hitler, which rose from being the leader to chancellor, and then the supreme leader.
Start of WWII:
WWII can actually be partly the Allies fault, due to mismanagement and fighting a defensive war (at first).
1) German Aggression: To solve some of the economic problems, Germany started to annex territories, build up it's army, and produce military equipment (all breaking the Treaty of Versailles, which could have led to the Allies breaking down his government, essentially stopping the war in the West).
2) Japanese greed: Japan soon joined the Axis Power, because Japan believed that they essentially "got the wrong end of the stick". Japan joined the war on the Allied side during WWI, and seized many German colonies in the Pacific. Expecting more, they helped the Allied side. However, during the treaty, the Allies did not give Japan what they wanted, leading to Japan hatred, and them joining the Axis in the Second World War.
3) Italy: Italy essentially also had a coup'de'tat, as Mussolini was able to take over the government. He soon joined the Axis side, completing the major powers. Mussolini started his aggression by building up his military (which was technically legal, as they were not limited under the Treaty of Versailles), and taking over African colonies.
Effects of the War:
1) During the war, we saw large technological advancements in many branches. One of the largest effect was the creation of nuclear weapons, which was what made the Cold War extremely scary for the duration of the period.
2) The governments of Germany & Italy were toppled and changed. Japan was able to keep parts of it's government, but was constantly monitored by the US, and parts were changed.
3) Rise of Super Powers: In the end of WWII, we saw the rise of two allies who will later turn bitter enemies because of their economic class differences, the U.S.S.R and the U.S.A. With the differences, and the hope to extend influence throughout the world (spheres of influence in a sense), they soon got into the Cold War.
