So the principles of warfare are: Do not depend on the enemy not coming, but depend on our readiness against him. Do not depend on the enemy not attacking, but depend on our position that cannot be attacked.
All people should disclose whether they submit to Allah or not. Those who are against us are our enemies and must be fought. That includes people who call themselves Muslims but who don't lead their lives as such - people who drink, who don't pray, who don't fast, who have constantly changing partners and who are unable to recite the Koran.
It is sufficient to estimate the enemy situation correctly and to concentrate your strength to capture him. There is no more to it than this. He who lacks foresight and underestimates his enemy will surely be captured by him.