Germany they started it ...
France and Belgium, Russia, Italy all saw heavy fighting in WWI.
In WWI, NZ was fighting for the Empire troops, and were in fact under the control of Brit commanders.
There were many different countries fighting WW1 like England, France, America, Belgium, Serbia and Russia.
A country can be involved in WWI without actually fighting. They can trade with only certain countries, they can send food and supplies out, and they can also raise money.
Alot of WW1's fighting occured in areas in France, Belgium, England, and Germany. You should really clarify your question if you would like a more specific.
having to do with allies and fighting against other countries
fighting in trenches
"Blighty" was a name used by English soldiers in WW1 when refering to England
It was terrible, spending all your time either in a dirty trench with rats or up on the front line fighting for your country.
on a battlefield
Treaty of Versailles