Before the war, people mainly used ships and boats, therefore, the people with the submarine could destroy these ships.
Submarines changed war. They played a big part of World War I and II.
Commerce warfare (Guerre de Course).
World War 1 introduced aerial warfare with airplanes and trench warfare
It sounds as if you are looking for submarines.
There were subs in the 19th century, but they were really a part of naval warfare from World War One to the end of the Cold War. If large scale war breaks out in the near future, expect submarines to be a huge factor on the ocean front.
Why don't you just Google "World War 1 Submarines?"
they used trench warfare and used powerful machineguns
trench was the most common but there was also airfighting and underwater in submarines ( then known as u boats)
Warfare in World War 2 are illegal and dangerous than warfare in World War 1.
World War I submarines were primarily used to locate and attack surface cargo and warships.
Warfare change from the Revolutionary War to present day.
Submarine warfare has existed since the early 1900's, and continues to this day. Though the last ship sinking by a submarine was in the Falklands War, the evolving missions of submarines have taken submarine warfare well beyond just sinking ships and submarines.