This concept actually corresponds with many of the original concepts of the US Special Forces as well as those of the French Foreign Legion.
The U.S. Marine Corps wants to form an Irregular Warfare Regiment (IWR) that would be a cross between the French Foreign Legion and the U.S. Army Special Forces. The IWR would have 4,200 troops and about 3,000 would be foreign born and selected because they were physically and mentally able to enlist and had language and cultural awareness skills the marines needed in various parts of the world. All officers and NCOs above the rank of E-5 (sergeant) would be U.S. citizens. If the program is established eventually many IWR officers and senior NCOs would be naturalized citizens. In effect, a foreign legion composed mostly of foreign volunteers seeking a quicker path to citizenship and able to meet Marine Corps standards.
Many IWR would be recruited overseas and after security screening would, if necessary, be sent to English language school where they would have to complete the course (and attain sufficient proficiency to handle military service) and then sent to boot camp. Once completing that they would be marines, serving a five year enlistment, and would then be sent to existing courses for intelligence, advising foreign forces, information warfare, counterinsurgency and security. All this would take nearly two years. Successful completion of all that would mean they would show up at the IWR with three years left. Upon successful completion of their five year enlistment non-citizen marines would become naturalized citizens. The IWR itself would be a light infantry unit with most of the troops expected to serve in small detachments with marine battalions or companies assigned to a foreign area. The IWR marines would be trusted translators and advisors who would be better able to work with foreigners whose language and culture they grew up in. In addition they are marines and that makes it easier for all marines in the vicinity.
The U.S. military has had experience with similar programs but all have suffered from problems with doing background checks for personnel who will be handling classified materials. The most recent such program, the 2008 MAVNI (Military Accessions Vital to the National Interest) was looking for qualified non-citizens able to provide needed language or medical skills. Some 10,000 non-citizens were enlistedfrom 2009 until the program was put on hold in 2017. There were problems with the security screening. There were not enough qualified people to do it and it was believed foreign intel organizations were seeking to use MAVNI as a way to insert agents. That was because as originally set up MAVNI was the quickest way for a non-citizen to get a security clearance and citizenship. The IWR program somewhat solves that problem because getting through marine boot camp and serving with other marines over an extended period has proved to be something foreign intelligence agencies deem too difficult to deal with.
Actually, an even earlier program, that was not particularly special at all, handled about 100,000 non-citizens with better results than with native born Americans. In the decade after September 11, 2001 it was found that non-citizens of prime military age (18-29) made up about 2.2 percent of the U.S. population, but 4 percent of military personnel. There are about 1.2 million non-citizens who are physically, mentally, and psychologically fit to serve in the military. These men and women are particularly attractive to the military because they tend to work harder, have fewer disciplinary problems, and often possess language skills and cultural knowledge that the military needs. But a major reason non-citizens are overrepresented in the military is that it’s an ancient tradition for a newcomer to gain membership in the tribe/kingdom/country via performing some dangerous service to gain recognition and acceptance.
In that decade the U.S. military enlisted some 70,000 non-citizens, about five percent of all recruits. The foreign recruits were tossed out during their first three months of service at half the rate of their native born counterparts. After three years of service 72 percent of citizens were still in uniform, compared to 84 percent of non-citizen troops. The foreign troops were more patriotic and worked harder than their citizen counterparts. Non-citizen troops had another incentive, as they could apply for citizenship sooner because of their military service. Any foreign recruit forced out for medical reasons (because of combat or non-combat injuries) could still obtain citizenship more quickly. Most foreign troops obtain citizenship as soon as they can while in the military because many jobs require a security clearance and only citizens can get one of those.
In the decade after 2001 some senior American officers urged the recruitment of more foreigners. Not just non-citizens with green cards but foreigners who were not residents of the United States. This brought forth protests from those opposed to, well, whatever. Historically, the American military has usually had a higher percentage of foreigners in the ranks than it does now. During the American Civil War about twenty percent of the Union Army was foreign born troops. There were entire divisions of Irish, Germans, or Scandinavians. For the rest of the 20th century the all-volunteer military continued to have a higher (than today) percentage of foreigners. Recruiting foreigners enabled the army to get more enthusiastic and capable recruits. Naturally they would have to speak acceptable English, just as resident foreigners in the United States or citizens from Puerto Rico must. The American military pay and benefits are competitive with U.S. civilian occupations but to many foreigners these pay levels are astronomical. The risk is low, as only about one in a thousand foreign born volunteers died in Iraq or Afghanistan. All that and you get to become a citizen of the United States after your four year enlistment is up. The only question was which line would be longer at American embassies, the one for visas or the one for military recruiting?
The United States is not alone in this acceptance of foreigners in the military. Take, for example, Britain. Two centuries ago Nepalese Gurkhas were first recruited into the British Indian army and then the British army. After India became independent in 1947, they too recruited Gurkhas for elite Indian infantry units. But service in the British army was considered a better deal. Britain has long recruited foreigners into its army and navy because there has always been a shortage of British citizens willing to serve.
Then there is the French Foreign Legion, which is supposed to be nothing but foreigners (except for the officers). But many French join, claiming to be from the French speaking parts of Belgium. No matter, if otherwise qualified the “Belgians” are signed up. In Italy, the Vatican (a small part of Rome that is an independent country controlled by the Roman Catholic Church) gets most of its security forces from Catholic areas of Switzerland. This is the Swiss Guard. While the French Foreign Legion dates from the 19th century, the Swiss have been serving as foreign mercenaries since the 15th century. But these contingents disappeared as better economic opportunities developed in Switzerland and mercenaries became less popular. Many other nations have successfully used foreigners in their armed forces. Not mercenaries but foreigners willing and able to serve next to the native born. It still works.