Tributes poured in from across the world on Sunday for Senator Lindsey Graham, the longtime U.S. lawmaker and foreign policy hawk who died unexpectedly at age 71.

“Senator Lindsey Graham, one of the greatest people and Senators I have ever known, is dead!” President Trump wrote in a post on Truth Social. “He was always working, and was a true American Patriot. Lindsey will be greatly missed!!!”

Mr. Graham, a South Carolina Republican, died on Saturday after a “brief and sudden illness,” his office said. Once one of Mr. Trump’s harshest critics, Mr. Graham had refashioned himself in recent years as one of the president’s most devoted backers.

Across Mr. Graham’s many political lives, his unyielding endorsement of American military intervention was perhaps his most consistent cause. Most recently, Mr. Graham carried the torch for the U.S. war against Iran, and encouraged aggressive support for Ukraine in its defense against the Russian invasion.

President Volodymyr Zelensky of Ukraine said that he had met with Mr. Graham just last week in Kyiv and was “deeply saddened” by his death.

“Lindsey was a true defender of freedom and the values that make our world safer,” Mr. Zelensky said in a post on X, adding that Mr. Graham had visited Ukraine 10 times since Russia’s full-scale invasion in 2022.

“We remained in constant dialogue, and I will miss our conversations. We met twice in just the past week.”

Mr. Graham was also a staunch defender of Israel, making numerous visits to that country. Israel’s prime minister, Benjamin Netanyahu, called him “a beloved friend.”

“Lindsey understood that the security of Israel and America are inseparable,” Mr. Netanyahu said in a post on X. “He devoted his life to defending America, strengthening our alliance and standing up for the free world.”

Mr. Graham shared a decades-long friendship with the late Senator John McCain of Arizona, another outspoken voice on foreign policy in the Republican Party who also advocated for an interventionist U.S. foreign policy.

“From the time he met my Dad they were fast friends and political comrades,” Mr. McCain’s daughter Megan McCain wrote on X, recalling his “acerbic quick wit” and fondness for storytelling.

“He was always full of stories that were always engaging and entertaining. He was literally never, ever boring.”