Czechoslovakia is the correct answer
Trench warfare is a type of land warfare using occupied fighting lines largely comprising military trenches, in which troops are well-protected from the enemy's small arms fire and are substantially sheltered from artillery.
Those serbs were convicted of War crimes because they approve the policy of ethnic cleansing in the war.
Ethnic cleansing refer to the policy in which they massacre a specific ethnic groups in their society. This practice deemed as really inhumane so they had to be convicted because of that
