The previous post was very strong. I would add that from an intellectual standpoint, one of the most profound effects of World War I was the embrace of fragmentation or disunity that embraced so much of the world outside of the First World War. The war was something where nothing really substantial positive or productive emerged. Europe was completely crippled from the emotional, political, and psychological costs of the War. Each nation was forced to rebuild from virtually nothing. At the same time, there was very little that was reaffirmed from the war. A conflict that was predicated upon nationalism, patriotism, and any other "-ism" that was seen as a form of structure was completely undermined by the destructive and brutal nature of the war. In the end, this psychological malaise was something that could be seen as a negative effect from WWI.
In my opinion, by far the most negative effect of WWI was WWII. The first war led pretty directly to the second.
The First World War led to a situation in which Germany (especially) felt the need for revenge and the need to upset the status quo. The Treaty of Versailles that ended WWI took much of Germany's territory away from it and imposed crushing financial reparations on it. These led to a desire for revenge. Germany also wanted to put an end to aspects of the treaty that prevented it from having a serious military.
By punishing Germany harshly, the Treaty of Versailles led Germany to have a motive for starting WWII. This makes WWII the main negative effect of WWI.
The negative effects of World War 1 are:
- The deaths of almost 10 million people and the wounding of 21 million others resulting in the destruction of an entire generation of predominantly young men.
- The devastation of the economies of many European nations and the creation of huge amounts of debt.
- The disillusionment and despair of War's survivors and a general sense of pessimism and insecurity worldwide.
- The Treaty of Versailles, the punitive provisions of which left a legacy of German bitterness that would lead the world into another catastrophic war within two decades.
- The Great Global Depression of the 1930s.
- The rise of fascist government in Italy, Spain, and Germany.
- The Second World War.