The "war to end all wars" was supposedly what is referred to as the first world war, or WWI. It is obviously not the case that WWI ended wars. This phrase, or something like it, started with H.G. Wells, I believe.
It never stopped being referred to as the great war as a matter of fact. People till this day still sometimes call WWI "The great War" and WWII "The War". Though I'm sure you have also heard people call WWI "The war to end all wars" as well.
Just as "Yank" referred to American soldiers in WWI and WWII, "Tommy" referred to any soldier in Great Britain's army.
During and after WWI, nobody knew there would be another huge war, and everybody certainly hoped there would not. WWI was called "The Great War", until WWII came along. Since WWII got going on a large scale, they have been referred to as WWI and WWII.
WWI tanks certainly weren't anything modern by today's standards. Even by the mid-1920s, they were obsolete. But the British were the first to use tanks, and the French were the first to use a turret mounted gun.
Also, it was called WWII because from 1914 -1918, the world fought another large war that was referred to as WWI ("the war to end all war").
American Involvement in WWI American Involvement in WWI
The first modern war. World War 1 was the Great War.
The First World War (also WWI).
The Great War (WWI) was the first of its kind, one that involved many nations.