WASHINGTON — In a victory for the Mashpee Wampanoag Tribe in its yearslong battle for sovereignty, a federal judge has ruled the Interior Department incorrectly applied its own guidelines in finding the tribe did not qualify for land-in-trust status.

U.S. District Judge Paul Friedman found that the department’s 2018 decision that the tribe was not under federal jurisdiction in 1934 was “arbitrary, capricious, an abuse of discretion and contrary to law.” He sent the case back to the department for “thorough reconsideration and reevaluation of the evidence.”

The judge also said the department could take no further action on disestablishing the tribe’s reservation until it correctly applied its guidelines on reconsideration.

“I’m feeling very happy,” Tribal Council Chairman Cedric Cromwell said Friday night. “We’ve been on a battlefield for a very long time in trying to protect my tribe and the Mashpee Wampanoag Tribe’s rights to continue having our homelands, our reservation lands for housing, education, health care, all the important services that a sovereign nation of the Mashpee Wampanoag Tribe should have of our own homelands.”

The Interior Department had taken 321 acres in Mashpee and Taunton into trust for the tribe in 2015, but under the Trump administration reversed itself in 2018. A federal judge in Boston ruled in favor of the reversal, and the tribe lost on appeal. The tribe also sued the secretary of the Interior in a Washington, D.C., court. That was the case it won Friday.

In late March, the Interior Department informed the tribe it would remove its land from trust status. The tribe asked the federal court to issue an emergency order that would postpone that decision. Friedman heard arguments on that request and on the lawsuit May 20.

The Interior Department must take a second look at the tribe’s case using the so-called M-Opinion, Friedman wrote, and evaluate all the evidence in concert rather than in isolation.

The M-Opinion sets guidelines the department had been using since 2014 to determine whether a tribe was under federal jurisdiction at the time of the Indian Reorganization Act in 1934 — a qualification for land-in-trust status. In March, the deputy solicitor for Indian Affairs, Daniel H. Jorjani, withdrew the opinion and issued a new four-step procedure for determining tribal eligibility.

If the department writes a positive record of decision using the M-Opinion, that could resolve the tribe’s other litigation over a proposed casino in Taunton, Cromwell said.

The tribe has until July to appeal that case, he said.

“Let’s say they write it positive, and it’s a positive record of decision, then we’re done. It’s over. That’s a success,” Cromwell said.

“Today was a big day for the Mashpee Wampanoag Tribe,” Cromwell said. “It was a victory. It was clear that the judge pointed out that the Trump administration did not follow the law in evaluating our land-into-trust application ... We hope to see justice continue to prevail, and we ask that everyone continue to stand with Mashpee.”