Underseaboats.
Submarine warfare is a military tactic used by many nations where submarines are used to attack enemy ships or as a direct deterrent. This was used extensively by the Germans during World War II.
During WW2, all the major countries involved used submarines.
In almost all navies during WWII, submarines were called "boats"...not ships. In Germany, a sub was called an "under-sea-boat", with German enthusiasts preferring to use the German spelling/pronunciation of "boot" for boat. Bottom line: "U-Boat submarine warfare..." is simply submarine warfare (Sub Warfare is what most boys in America grew up calling it).
Submarine warfare was abhorrent in the US during WW1. The British sank US ships trying to enter blockaded German ports. The same is true of US ships trying to enter blockaded British ports. However the Germans use submarine warfare to sink a USS Lusitania. That was the tipping for the US. Prior to then we were not wanting to get into "entangling alliances.
During the Treaty of Versailles
The Uboat was a submarine made by the Germans
Most likely Germany, they had the U-boats (underwater boats)
What political group wanted to take power in Germany during WW1.
Two actions that upset the Americans during World War I were Germany's unrestricted submarine warfare and the interception of the Zimmermann Telegram. The unrestricted submarine warfare led to the sinking of civilian and commercial ships, including the Lusitania, which resulted in American casualties. Meanwhile, the Zimmermann Telegram revealed Germany's proposal to Mexico to join the war against the U.S., further inflaming American public sentiment and prompting the U.S. to enter the conflict.
Gas was first used in warfare during the Second Battle of Ypres in 1915 by the Germans.
The Germans faced France and French colonial forces, Britain and Britich colonial forces, and, eventually, the Americans on their Western Front in trench warfare. Austrian and Germans faced the Russians for a time on the Eaastern Front, some of that in trench warefare.