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They come from other countries, but they fight for Israel. And sometimes die for it.

They are known as lone soldiers — thousands of volunteers from around the world who join the Israeli Defense Forces, often in combat units, but have no family inside the Jewish state. They typically serve a year and a half. They train alongside Israeli citizens, and today they fight next to them in Israel’s incursion against Hamas in the Gaza Strip.

“The foreignness drops away real quick,” said Adam Harmon, who moved to Israel from New Hampshire out of college and joined the IDF in 1990, when Israel was fighting a Palestinian uprising and an ascendant Hezbollah.

The main difference between himself and his fellow soldiers, he recalled, was that he was 22 and they were 18 — just beginning their compulsory service and focused on girls, soccer, cars.

Harmon had been on summer trips to Israel and moved because “I just felt I belonged to that place, and it belonged to me.” He signed up for the IDF because he wanted to share the burden of service. He later wrote a book, “Lonely Soldier,” about his experience.

An estimated 2,000 soldiers in the IDF today are from the United States, and on Sunday two of them lost their lives — a 24-year-old from Southern California and a 21-year-old from Texas. Their families said they both had a passion for Israel.

Former lone soldiers say they sometimes drew curious looks from Israeli soldiers, for whom service is mandatory. But they described feeling as strong a sense of patriotism for Israel as they do for the United States.

“The bottom line is, I’m part of the Jewish people,” said David Joel, who grew up outside Atlanta and said he was inspired to serve after he narrowly missed being killed in a terrorist attack in Jerusalem when he visited with a friend in 2000.

“We decided that instead of going away we were going to donate something to the country,” he said. “We were Jewish, and we believe in the Jewish country. At the end of the day, it’s our nation.”

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Joel served in the infantry as a heavy machine gunner during the Palestinian uprising known as the Second Intifada, from 2000 to 2005. He served in Ramallah, in the West Bank, and in Bethlehem, where his unit arrested what he described as terrorists.

Joel gained Israel citizenship in 2004, and today lives in the city of Shilo, outside Jerusalem. During an interview with NBC News, he got an alert on his phone about possible nearby incoming rockets from Hamas.

There are about 6,000 lone soldiers in the IDF today, said Josh Flaster, director of the Lone Soldier Center, an Israeli nonprofit. That figure includes not just lone soldiers who come from other countries but also Israeli orphans and those from broken homes.

About a third of those are from the United States, but sizable contingents are also from France, Russia and Argentina, Flaster said.

The IDF overall numbers about 176,000 active service members, according to Jane’s, which keeps military statistics.

Asked whether the ranks of lone soldiers rise at times when Israel is involved in heavy fighting, like its battle against Hamas, Flaster pointed out that Israel had also fought armed conflicts in 2006, 2008, 2009 and 2012.

“The crazy people are coming to the top” in the Middle East, he said, so “young Jewish guys and girls around the world who care about Israel, who want to defend Jews and civilians, they come.”

Two who came from the United States were Max Steinberg, 24, and Nissim Sean Carmeli, 21. They were among 13 Israeli soldiers and at least 65 Palestinians killed on Sunday in Gaza in the first major ground battle of the Israeli incursion.

Steinberg was a sharpshooter and Carmeli a sergeant.

Steinberg’s father, Stuart, told The Associated Press that his son visited Israel for the first time on a Birthright trip in June 2012. When he came home, he told his parents that he would go back to join the IDF.

“I always wanted to serve in the military in one capacity or another, whether that’s America or Israel.”

“He was completely dedicated and committed to serving the country of Israel,” the father told the AP. “He was focused, he was clear in what the mission was, and he was dedicated to the work he needed to be doing.”

Carmeli’s high school in Texas, St. Joseph Academy, remembered him as a “very devout and dedicated young Jewish man” and sent prayers to his family. Carmeli grew up on South Padre Island and moved to Israel to finish high school.

Rafe Kaplan, who lives in Milford, Connecticut, and is preparing to enter medical school, joined the IDF in September 2012, right after college, and served as a paratrooper. He returned to the States in January.

He had visited Israel but didn’t pursue citizenship because it “culturally just wasn’t for me,” and he considered joining the U.S. Marines, but that was a four-year commitment and he had medical school to think about.

“I always wanted to serve in the military in one capacity or another, whether that’s America or Israel,” he said. I love both countries. And I’ve always respected soldiers. I’ve always wanted to serve my country.”

Kaplan’s old unit is in Gaza today, and he said he worries for them.

“I know they were trained well,” he said. “I know that they’re doing the best they can.”