The art of war is extremely complex and there is no way to cover everything in one Lemmy comment.

However, I can summarize: An attacker or defender simply needs to prevent the opposing side from being able to support a war.

While there are thousands of different things that can support war, it usually boils down to raw manpower, food/supplies, weapons, energy, logistics and communications. Failing to defend, or not having the capability to replenish/repair those things is usually a quick game-over as those items are highly dependent on the other. Anything that supports those key items is a target of the enemy, so those are the things that are stockpiled, fortified or should be rebuilt quickly.