War begins in Belgium describe what happened what happened: Deadly weapons of WWI give some examples of what weapons were introduced in WWI:

U-boats What were they, who did they come from and what did they do:

1. Germany declared war on France. To avoid the French fortifications along the French German border, the troops had to cross Belgium and attack the French army by the North. Of course, Belgians refuse to let them through, so the Germans decided to enter by force and invaded Belgium. The German mistreatment of Belgian civilians during the invasion was tragic. civilians lived in a nightmare during the 4 years of occupation. The numbers speak for themselves almost 9,000 civilians were deliberately killed by the Germans and 6,453 of them were killed during the first week of occupation.

2. there were a lot of deadly weapons that were introduced here is a list of them: barbed wire, machine guns (most of them were based on Hiram Maxim's 1884 design), artillery, poison gas, tanks, and rifles.

3. A U-boat, German abbreviation of undersea boat, essentially a German submarine. The destruction of enemy shipping by German U-boats was a spectacular feature of both world war I and world war II. Wilhelm Bauer was the creator of it and it was designed in 1850s. They created it to improve their odds.