Answer:
Everyone knew it was coming
Step-by-step explanation:
WW1 was caused by the assassination of the Austrian-Hungarian Archduke Franz Fernand in Serbia, which had previously declared independence from the Austrian-Hungarian empire. This led to an escalation and the Austrian Hungarian Empire declared war on Serbia, this led to the United Kingdom and France also declaring war, but on the Austrian-Hungarian Empire. Germany, an ally of the AH Empire, declared war on the Serbian and the allies. This led to the creation of the Allies (UK, France, and Russia) and the Central Powers (Germany and AH Empire, later the Ottoman Empire would join). The main goal was to conquer more territory and destroy the allies, as in every war. However, this war was known as "The Great War" back then. I should mention that the US did not join until it was known that Germany sent the Zimmerman Telegram in 1917. Fast forward the AH Empire surrendered, and finally, Germany surrendered as well. This led to the Treaty of Versailles, which punished Germany a lot. It had to give land back to France, pay a huge amount of money it didn't have, reduce the military to only 100,000 men, no air force, and Poland was created out of its borders. Many people say that this is the reason why WW2 began in the first place, because it caused the rise of Hitler. aaaaaaaannddd here we go to WW2
WW2 was caused because the invasion of Poland by Nazi Germany. Hitler made an alliance with Stalin (Leader of the Soviet Union) that they would both invade Poland so Hitler would not end up fighting a war on two sides. The United Kingdom and France declared war on Germany. Hitler's main goal was Lebensraum meaning that he needed to conquer the world so he could increase the Aryan race. Hitler invaded France in 1940, and just like that, France fell to the Germans. Then, Hitler invaded the Soviet Union in 1941. The Soviets were pushed back, and the Germans were at the gates of Moscow. However, winter came, and the germans had to wait until the spring. The Soviets pushed the germans out of Moscow. Fast forward the US declared war on Japan after the attacks of Pearl Harbor, and officially enter WW2 in 1941. In 1944, the Americans led an invasion of France to liberate them. Fast forward to 1945, Hitler committed suicid in his bunker. WW2 ended in Europe on May 8, 1945 as the Soviets captured Berlin. The Americans dropped 2 bombs on Japan, one in Hiroshima, and one in Nagasaki. It officially ended on September 1, 1945 as the Japanese surrendered. This led to the rise of 2 global superpowers, the United States, and the Soviet Union, which will start the Cold War, which lasted 1947-1991. The Soviet Union would collapse in 1991, but thats a story for another time :)
Allies: USA, UK, France, Soviet Union, among others
Axis Powers: Nazi Germany, Italy, Japan, among others
There's more to it, but I gave you the basics. I recommended watching Oversimplified for further detail, also he is very entertaining and funny.