to destroy enemy ships and to kill allies
The Germans then threatened to use only Uboats for war and blew up a few ships.
because of the alliances the US had made previous to the start of the war along with the threat of the German Uboats attacking US civilians and property.
To increase public support for the war
Sending troops to Europe was done for the purpose of creating a strong military in World War 1. There is strength in numbers.
To simply end the war for world war 1, for world war 2 it was end the war and take down the Nazis
The assassination of Franz Ferdinand (not the band)
water transport
Scouting & sinking ships.
observation balloon
to divide the Ottoman Empire after the war
Hmmm, there were rather more than two.
Killing people, the same as any other era's handguns.