The Supreme Court on Thursday let the Trump administration end temporary protections that have allowed more than 350,000 people from Haiti and Syria to live and work legally in the United States.

The decision regarding the Temporary Protected Status program applies only to people from those two countries, but it will have implications for all 1.3 million T.P.S. holders in the United States, who come from more than a dozen nations.

T.P.S. allows foreign nationals to remain in the country for up to 18 months when a crisis such as an earthquake or armed conflict makes returning to their home countries unsafe. It was passed with bipartisan support by Congress and signed into law by President George H.W. Bush in 1990.

But the measure, which can be renewed by the federal government, has become all but permanent for recipients from some nations where crises have spanned many years.

Justice Samuel A. Alito Jr. said in Thursday’s decision that federal law prohibited courts from second-guessing an administration’s determination of which nationalities deserve protection and which don’t.

The Trump administration remains locked in litigation over whether it can end protections for more than one million people from Afghanistan, Cameroon, Ethiopia, Honduras, Myanmar (formerly Burma), Nepal, Venezuela, Somalia, South Sudan and Yemen. Protections for four countries — El Salvador, Lebanon, Ukraine and Sudan — have not been affected.

The Supreme Court’s ruling is likely to narrow the chances litigants from countries besides Haiti and Syria have of winning cases that seek to preserve their own T.P.S. protections. And some T.P.S. holders and their supporters fear it will embolden administration officials to end the program for additional countries.

What does it mean for T.P.S. holders?

T.P.S. holders are scrambling find other forms of immigration protection. Many have already been applying for asylum, work visas or other types of authorization to live and work in the United States. But the pathways are narrow, and petitions can take years to process.

For many, the path to asylum will be an uphill battle, if it is viable at all. Immigrants must petition for that relief within a year of arrival, and qualifications are rigid. The Trump administration has restricted eligibility and pressured immigration judges to deny the claims. They are granting asylum in less than 10 percent of cases, far lower even than in Mr. Trump’s first term.

Instead, many T.P.S. holders must decide whether to make exit plans or remain without authorization. Some have been looking for work and legal passage into other countries, such as Canada. Others, such as Somalian T.P.S. holders, may be forced to return to countries where war or conflict rages.

Will there be immediate deportations?

U.S. Immigration and Customs Enforcement officers will be able to place in removal proceedings any T.P.S. holders whose protections have expired or ended. Immigrants can then be placed into a detention system that encompasses more than 200 facilities.

Immigrants can contest deportation in court, but federal officials have hindered them by fast-tracking cases, slashing appeals deadlines and making it harder for people to obtain bond and seek counsel.

What does it mean for employers?

Employers — including hospitals and hospices, construction companies and retail stores — will have to immediately fire Haitian and Syrian workers whose sole authority to work comes through the program.

Some T.P.S. recipients might have the right to work through a different immigration status, such as asylum or visa petitions. And some businesses may attempt to secure work visas for employees, but those are costly and can take years.

The Supreme Court’s decision will fall most heavily in the health care industry, where Haitians in particular have been concentrated, especially in elder care facilities. Health care has been the American economy’s fastest-growing industry for years now, and the vacancy rate is substantially higher than it is for others, so finding new workers on short notice will be very difficult.

“Providers will have to make tough choices, which could include limiting nursing home admissions, closing units or turning away requests for home health and home care, until the vacant jobs are filled,” said Katie Smith Sloan, chief executive of LeadingAge, an association of nonprofit aging services providers.

Over the past year, employers have been scrubbing workers’ documents to prepare for audits or raids by federal immigration enforcement. Some have preemptively fired employees with T.P.S., given their tenuous status.

If they don’t do so after Thursday, they risk fines, although penalties for employers are usually minor unless officials find evidence of repeated, intentional violations.

What does it mean for students and schools?

Many T.P.S. holders are researchers, administrative staff, adjunct faculty or vital service workers. Colleges and universities will be required to end the employment of any Haitian and Syrian T.P.S. holders.

Many students will be stripped of work permits and cut off from income. In several states, the end of their status will mean they will lose in-state tuition rates or state-funded financial aid, meaning some may be priced out of their education.