TEL AVIV—Israeli decision-makers plan to resume the Gaza war in four to six weeks with overwhelming force, sending in tens of thousands of troops to conquer the entire strip in a single coordinated offensive against Hamas. Incoming military chief of staff Eyal Zamir has, at the direction of Prime Minister Benjamin Netanyahu and defense minister Israel Katz, started developing the plan, according to several current and former Israeli officials with knowledge of high-level discussions. Under the plan, Israel will deploy more troops to Gaza than it has to this point in the war—over 50,000—before relocating the civilian population to humanitarian...