Israel’s former military chief Benny Gantz declared victory Thursday in Israel’s election and rebuffed Prime Minister Benjamin Netanyahu’s calls for power sharing, as the country braced for weeks of political uncertainty and geopolitical tensions rise in the Middle East.

Mr. Gantz vowed to lead a broad coalition from across Israel’s political spectrum, known as a unity government, ending several years of right-wing and religious majorities led by Mr. Netanyahu. The prime minister conceded Thursday that he would be unable to...