Once really regarding WW2, WW1 is a very complicated war, the events that led up to it, etc. There's a lot of bad involvement of many countries there, there is no singular blame. The way treaties were setup pulled in countries into conflicts. Because of this, when conflicts broke out between different countries, others had to be pulled in due to these treaties, so it became a huge mess of countries being pulled in and fighting others that they weren't initially. Lets not forget about the imperialism that brought tensions too. Germany didn't cause the events that led to WW1, Germany just escalated it when it was already on the brink. Austraia-Hunary declared war on Serbia.
Due to the treaty between Russia and Serbia, Russia had to help out and they were doing heavy mobilisations. This was a result of Archduke Franz Ferdinand being assassinated (and other attempts by Black Hand terrorist group) by a Serbian natationalist. Germany then declared war on Russia. Basically because of all the crazy treaties/clusterfuck, countries were beginning to be pulled in, resulting in WW1. Saying Germany caused WW1 is ignoring too many of the events and nature of the world in terms of alliances/treaties at the time, plus competition of imperialism that brought conflicts, etc. Many historians argue that the true cause was the tension of imperialism amongst all the many countries, the situation of WW1 has too many hands involved to give singular blame.
Also, WW2 was also partly as a result of how Germany was treated after WW1, the economy, the Germany people feeling ashamed of being German, giving rise to nationalism by a leader like Hitler, promising to bring the country out of the dire economy/poverty it was in which he did, and he made Germans proud. The way Germany was treated after WW1 was in the nature/treatment that Germany caused everything for WW1 which wasn't true, Germany took the biggest hit and nature of singular blame when it wasn't its entire fault, again - there is no singular blame in WW1 but Germany was treated as if it were.
Post-WW2, there was a reason why the allies did not make the same mistake of post-WW1, it's why Germany and Japan for example were helped to be rebuilt, allowed to exist better than how it was treated post-WW1. The reason Germany was "allowed to exist as it does today" is because leaders/politicians did not want to make the same mistake after WW1. Germany is one the most peaceful countries in the world today, same goes for Japan for the most part (I guess Abe and his cronies are trying to change that).
