Overnight Defense: US aircraft carrier staying in Mideast in abrupt reversal | DC Guard activated ahead of pro-Trump protests | 10 former Defense secretaries speak out against military involvement in election dispute

Happy Monday and welcome to Overnight Defense. I’m Rebecca Kheel, and here’s your nightly guide to the latest developments at the Pentagon, on Capitol Hill and beyond. CLICK HERE to subscribe to the newsletter.

THE TOPLINE: The USS Nimitz aircraft carrier isn’t leaving the Middle East after all.

Last week, acting Defense Secretary Christopher Miller announced the Nimitz was coming back to the United States in what was seen as a de-escalatory signal to Iran.

But on Sunday night, Miller abruptly announced he changed his mind.

“Due to the recent threats issued by Iranian leaders against President Trump and other U.S. government officials, I have ordered the USS Nimitz to halt its routine redeployment,” Miller said in a statement. “The USS Nimitz will now remain on station in the U.S. Central Command area of operations. No one should doubt the resolve of the United States of America.”

U.S. officials have been worried Iran or its proxies would launch an attack to mark the anniversary of the U.S. drone strike that killed Iranian Gen. Qassem Soleimani. That anniversary passed Sunday without any such attack, but U.S. officials are still on alert.

What happened: Miller didn’t specify what threats he was referring to.

But the head of Iran’s judiciary, Ebrahim Raisi, seemed to issue an implicit threat to Trump on Friday when he said no one involved in Soleimani’s death will be able to “escape law and justice” even if they were a U.S. president.

Several news outlets reported Monday that Trump ordered Miller to reverse his decision.

Nuclear tensions: Meanwhile, Iran announced Monday that it resumed enriching uranium to 20 percent purity in its latest breach of the 2015 nuclear deal.

Iran began breaching the deal step-by-step in 2019 as Trump significantly tightened sanctions after withdrawing the United States from the deal the year before.

Twenty percent enrichment is still under weapons-grade. But once it reaches that level, it takes much less time to enrich to 90 percent, which is considered weapons-grade.

President-elect Joe Biden has said he would return the United States to the nuclear if Iran returns to full compliance.

And in a separate development that also could further inflame tensions, Iran said Monday that its Revolutionary Guards Corps seized a South Korean-flagged tanker in the Persian Gulf and detained its crew. State media said the Guard captured the tanker over allegations it was polluting the water with chemicals. Seoul demanded the ship’s release.

DC GUARD ACTIVATED AHEAD OF PROTESTS: National Guard troops are being activated in Washington, D.C., this week as thousands of Trump’s supporters are expected to arrive in the nation’s capital to protest the results of the presidential election amid Congress’s certification of the Electoral College vote.

Roughly 340 Guardsmen will be activated from Jan. 5 to 7 to help with crowd control at local subway stations and with street closures at several intersections to ensure pedestrian safety, the D.C. National Guard said in a news release Monday.

D.C. Mayor Muriel Bowser (D) and Christopher Rodriguez, director of the D.C. Homeland Security and Emergency Management Agency, requested the Guard’s support ahead of this week’s demonstrations.

“At the request of Mayor Muriel Bowser, the District of Columbia National Guard is in a support role to the Metropolitan Police Department (MPD) which will enable them to provide a safe environment for our fellow citizens to exercise their first amendment right to demonstrate,” Maj. Gen. William J. Walker, commanding general of the D.C. National Guard, said in a statement.

“Our main mission is augmenting select traffic control points and metro stations identified by MPD,” Walker added.

SecDefs speak out: As Trump’s supporters agitate for overturning the results of the election, all living former Defense secretaries are urging the military to stay out of it.

In an op-ed for the Washington Post, the 10 former officials — including Trump’s two Pentagon chiefs, James Mattis and Mark Esper — wrote that “the time for questioning the results has passed.”

“Our elections have occurred. Recounts and audits have been conducted. Appropriate challenges have been addressed by the courts. Governors have certified the results. And the electoral college has voted. The time for questioning the results has passed; the time for the formal counting of the electoral college votes, as prescribed in the Constitution and statute, has arrived,” they wrote.

“Efforts to involve the U.S. armed forces in resolving election disputes would take us into dangerous, unlawful and unconstitutional territory,” they added.

Former Defense Secretary William Perry tweeted that op-ed was Dick Cheney’s idea. Eric Edelman, a former diplomat and Defense official, told the Post the piece started as a conversation between himself and Cheney and that he solicited participation from other Defense secretaries after Cheney expressed interest in co-writing an opinion piece.

ICYMI OVER THE HOLIDAY — CONGRESS OVERRIDES NDAA VETO: Congress handed Trump the first and likely only veto override of his presidency late last week with the annual defense policy bill.

The fiscal year 2021 National Defense Authorization Act (NDAA) became law after the Senate voted 81-13 to override Trump’s veto during a rare New Year’s Day session. That followed the House’s 322-87 override vote earlier in the week.

Just seven of the Senate’s 52 GOP senators voted to uphold Trump’s veto: Mike Braun (Ind.), Tom Cotton (Ark.), Ted Cruz (Texas), Josh Hawley (Mo.), John Kennedy (La.), Mike Lee (Utah) and Rand Paul (Ky.).

Those seven were the same Republicans who voted against the final NDAA last month, meaning Trump didn’t pick up any Senate GOP support to try to prevent the override after vetoing the bill.

ON TAP FOR TOMORROW

The Center for Strategic and International Studies will host an online event with Federal Communications Commission Chairman Ajit Pai 5G and national security at 3 p.m. https://bit.ly/38dVV5F

ICYMI

— The Hill: Saudi Arabia agrees to open borders with Qatar in sign of easing rift

— The Hill: Five top challenges for Biden on defense

— The Hill: Biden VA pick faces ‘steep learning curve’ at massive agency

— The Hill: Opinion: On anniversary of downed Flight 752, it’s time to hold Iran accountable

— The Hill: Opinion: China backs Iran in times of crisis

— Wall Street Journal: Marines prepare for rising challenge from China’s military with island training

— New York Times: What to know as troubled Afghan peace talks enter a new phase

— Associated Press: After pardon, Blackwater guard defiant: ‘I acted correctly’

— Air Force Times: Air Force colonel, charged with sexual assault, to face court-martial