John Sena always wanted to be a soldier. His uncles served in the U.S. Navy and told him stories of their travels. His mother worked at a restaurant near a recruiting station during his childhood and he remembers seeing Marines in their dress uniforms. When he grew up, he thought, he would be just like them.

In January, 2016, Mr. Sena got his chance. He enlisted in the U.S. Army to become an active-duty soldier and swore allegiance to the U.S. Constitution, promising to protect it from all enemies, foreign and domestic.

Today he is still waiting to attend basic training and his whole future in the U.S. is in question.

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Mr. Sena is among the 800,000 people in the U.S. who have benefited from Obama-era protections for people brought to the country illegally as children (this group is often referred to as "DREAMers" after the name of a bill that sought to legalize their status). For Mr. Sena, the protections provided an opening to pursue his lifelong ambition of becoming a soldier.

Last month, U.S. President Donald Trump announced that he was ending the program, known as Deferred Action for Childhood Arrivals, or DACA. Starting early next year, about 2,000 DACA recipients a day will lose their status, meaning they will no longer have the ability to work legally and will no longer be shielded from deportation.

Mr. Trump's move to eliminate DACA ignited a firestorm of criticism. Both Democrats and Republicans in Congress have voiced sympathy for DACA recipients, many of whom can't recall living anywhere except the United States.

Yet efforts to reach a legislative solution have made little progress: Last week, the Trump administration said it would require funding for a border wall with Mexico, among other hardline demands, in exchange for forging a solution for DACA recipients. Democrats have refused to meet those conditions.

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Now the hundreds of thousands of DACA recipients could lose everything they have worked to achieve. Since the program was established by executive action in 2012, they have pursued studies and careers, bought homes and started families. The Migration Policy Institute, a non-partisan think tank, estimates that 14,000 DACA recipients are employed in health-care positions, while 9,000 are teachers.

Mr. Sena, 21, is part of an even more rarefied group. He is one of fewer than 900 DACA recipients who enlisted in the U.S. Army through the only avenue open to them: an initiative to recruit people with strategic language skills or medical expertise.

Born in the Philippines, Mr. Sena was brought to the United States at the age of 10 and his family settled in California. In high school, he played basketball and became a local wrestling champion. During his senior year, a recruiter for the Marines visited his school and asked Mr. Sena whether he was interested in signing up. "Hell yeah," Mr. Sena responded.

That evening, he asked his mother for the documents the recruiter required: a birth certificate, a high school transcript, a U.S. passport and a social security card. His mother could give him only the first two. "I started giving her looks, like, 'What happened to the other two?'" he recalled. "That's when I kind of found out … we're not supposed to be here."

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Dejected, Mr. Sena shelved his dream of joining the military. He began studying nursing at a community college while working at a supermarket. There, a colleague told him about the MAVNI program, which was launched in 2009 (the acronym stands for Military Accessions Vital to the National Interest). The program was open to non-citizens, including DACA recipients, with specialized skills. Mr. Sena and his twin brother, both proficient in Tagalog, considered a strategic language, rushed to join up.

"I always believed God did things in mysterious ways," said Mr. Sena. When he enlisted in the U.S. Army in early 2016, it was "as if God gave me another chance."

Last year, the U.S. government suspended further recruitment to the MAVNI program and instituted a new regime of extensive background checks for those who enlisted under its umbrella. So instead of starting basic training, Mr. Sena and hundreds of other DACA recruits have been stuck in limbo. (Normally, non-citizen soldiers become naturalized after they complete basic training.)

Margaret Stock, a lawyer and retired lieutenant colonel who helped found the MAVNI program, said the result has been "chaos" and a cruel prospect for people such as Mr. Sena. If a person's DACA status expires before all the background checks are complete, "then you get kicked out of the military and then you get deported after having spent two or, in some cases, three years of your life waiting," said Ms. Stock.

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Meanwhile, two times each week, at 7 a.m. or earlier, Mr. Sena goes to his recruiting station for physical training, where he is sometimes asked to lead exercises for newer recruits. His army recruiters are apologetic about his situation but unable to change it.

Mr. Sena's hope is that U.S lawmakers will overcome their past failures to pass some version of the DREAM Act, a proposed piece of legislation that would give people like him a path to U.S. citizenship. Then he could enter the military through a more traditional route – by applying to an on-campus officer training program at a university.

His enthusiasm for a military career remains undimmed, even as the likelihood of achieving that goal diminishes. "I've always known," said Mr. Sena. "It's what I've always wanted."