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Because, as America has learned in the Middle East, even with the biggest and best armies in the world, you cannot win against an enemy that is indistinguishable from innocent civilians UNLESS you are willing to accept those innocent civilians dying as collateral.
In historical times, the Roman Empire for example could just glass entire countries of people and not give a fuck, so conquering was simple. Now, we have to care about "human rights" and muh innocent civilians, making it impossible to truly conquer anywhere anymore.
If the British government didn't care about killing Irish civilians, they could have steamrolled the entire island in a week. But they did, so they didn't.