Which group did Israel declare war on that is based in Gaza?

Israel declared war on Hamas, which is a militant group based in Gaza. This group is known for its long-standing conflict with Israel, engaging in various forms of armed resistance and terrorist attacks. The designation of war against Hamas typically stems from their significant role in hostilities between Israel and Palestinians, particularly with their governance of the Gaza Strip since 2007 and their history of launching rocket attacks into Israeli territory. This context highlights Hamas as a primary actor in the ongoing conflict and the primary target of military actions declared by Israel.

While Hezbollah, Islamic Jihad, and the Al-Qassam Brigades also play roles in the regional dynamics and have engaged in conflict with Israel in different capacities or during specific events, Hamas is explicitly recognized for its direct governance and the frequency of its confrontations with Israeli forces, making it the correct answer in this context.