The Supreme Court agreed on Oct. 1 to take up one of the most consequential immigration battles of the Trump presidency: whether federal agents can lock up millions of undocumented immigrants without ever giving them a chance to ask for bond, even when they have lived in the United States for years and have no criminal record. The decision to hear the case puts the justices on a path toward a landmark ruling on a policy that has already produced tens of thousands of detentions across the country — and that lower courts have mostly struck down. At the heart of the dispute is how the government reads a 1996 immigration law. For decades, that law was understood to require mandatory detention mainly for people picked up at or near the border. Immigrants arrested in cities and towns where they were already living, by contrast, could ask an immigration judge to release them on bond, provided they could show they were not a danger to the public and would appear at future court proceedings. The Trump administration reinterpreted the statute to cover virtually everyone in the country illegally, ending those bond hearings. Administration officials say the change keeps undocumented immigrants from evading deportation. After a divided federal appeals court based in Missouri ruled for the administration in March, then-Attorney General Pam Bondi wrote on social media that the law was clear and had simply not been enforced. Opponents of the policy call it an unlawful attempt to pressure immigrants into leaving the country on their own, including people with long-pending citizenship applications and those with American-born children. "The policy at issue in these cases is truly a radical break with the law," said Michael Tan, an attorney with the American Civil Liberties Union, which is defending the immigrants. The legal fight has split the appeals courts. While one other court besides the Missouri-based panel has backed the administration, nine have ruled against it. In August, a divided appeals court based in Massachusetts held that immigrants already inside the country have a right to bond hearings after being picked up by U.S. Immigration and Customs Enforcement. Writing for the court, Judge Lara Montecalvo noted that five presidential administrations over three decades had applied the disputed statutes the same way the court's majority read them today. If the Supreme Court sides with the administration, millions of immigrants could become subject to mandatory detention. If the challengers prevail, bond hearings would return as a check on the government's power to detain people while their cases move through the immigration courts. The case is not the only immigration fight facing the justices this term. Just a day earlier, on Sept. 30, the court allowed the administration to restart deportations to countries other than immigrants' own while that legal fight continues — a sign of how central immigration has become to this term's docket. In December, the justices will hear arguments on whether the government can send immigrants to third countries without first letting them argue they would face persecution or torture there. Decisions in both cases are expected by the end of June, when the term closes. For now, the detention policy remains in force, and every new arrest under it can end in a detention cell with no hearing on release. Immigrant advocates say that reality has turned routine enforcement stops into indefinite lockups for people with deep roots in the country. The administration counters that without the tool, deportation orders would go unenforced. The justices' answer, when it comes next year, will settle which reading of the 1996 law governs — and how much freedom the government has to jail first and ask questions later.