In 1984, Oceania is in a constant state of war. While the enemy is subject to change, the purpose of warfare in Oceania remains the same, as Emmanuel Goldstein argues in his Theory and Practice of Oligarchical Collectivism. War, he says, is necessary to subjugate the people and to maintain the Party's power:

The primary aim of modern warfare…is to use up the products of the machine without raising the standard of the living.

In other words, the Party uses the war as justification for claiming the surplus goods that are produced in Oceania. These goods are directed towards the war effort instead of towards the people and this keeps the general standard of living low. This benefits the Party because it keeps people in a permanent state of dependence.

War, therefore, is one of the Party's most important political tools and this is why it is and must always be perpetuated.

Oceania is in a constant state of war. The enemy nation may shift from one country to another, but the state of war continues.

Early in the novel, Winston recalls that:

Oceania was at war with Eurasia and allied with Eastasia yesterday, and not vice versa.

War is a tool of the state in this novel, helping the government to control its population through fear, blind hate, and nationalism. As a post-World War II novel, Orwell's book reflects a fear that perpetual war may become a reality in which citizens are manipulated into obediance, made subject to sweeping fears of violent outbreaks or attack. 

Winston reflects on the state of war in the narrative and receives a full account of the government's position from O'Brien.