I'm assuming you're referring to mutinies by soldiers that occurred during The Great War, which later became known as World War I.
Men of the French army essentially decided to stop obeying orders and would not participate in any more futile attacks on the enemy.
An especially costly assault was launched by General Robert Nivelle, Commander of the French Army, in April 1917. 120,000 French soldiers lay dead after ten days and no ground had been gained. After that, various regiments of the French army began to refuse to obey orders for further attacks. About half of the French army ultimately participated in the mutiny.
In Russia, soldiers mutinied by participating in the Russian Revolution that toppled Tsar Nicholas II from power.
In February, 1917, the Cossack cavalry refused to shoot (as ordered) on protesting workers in Petrograd. The tsar's military governor in Petrograd issued orders to other military regiments to shoot any protesters who would not disperse, but many of the other soldiers refused such orders also, and mutiny spread. By March 2, 1917, Tsar Nicholas II was forced to abdicate his position as ruler of Russia.
