Trump mulls options on North Korea, Syria; Shakeup at NSC; Marines want to retire jets quickly; How to fill America’s missile-defense-sensor gaps; and just a bit more...

A day before President Trump’s big day with Chinese President Xi Jinping at Mar-a-lago, his National Security Advisor Lt. Gen. H.R. McMaster rang his South Korean counterpart to say “Washington remained committed to the deployment of the Terminal High Altitude Area Defense system in South Korea,” Reuters reports.

Trump himself called Japanese Prime Minister Shinzo Abe also on Thursday to say all options for responding to North Korea’s possibly growing nuclear program remain on the table.

U.S. Pacific Fleet Commander Admiral Scott Swift, speaking from Tokyo this morning, said a military response is among those options. “That decision would be up to the president," he told reporters. "The military was always an option."

We have a little more info on North Korea’s most recent rocket launch Wednesday morning: “U.S. officials said the missile launched on Wednesday appeared to be a liquid-fueled, extended-range Scud missile that only traveled a fraction of its range before spinning out of control,” Reuters reports.

Early U.S. and South Korean assessments “indicated it was an advanced KN-15 medium-range missile, whose first known test by North Korea was in February,” AP reports. “But unlike the KN-15, which uses solid fuel, the missile fired Wednesday used liquid fuel and was fired from a fixed location, rather than a mobile launcher.” More—including one analyst’s guess that the North may attempt to conduct another attention-grabbing nuclear test during the Trump-Xi summit—here.

The North could also try to work in a test on April 15, “an important national anniversary in North Korea,” The New York Times reports. “That timing would also embarrass Mr. Xi just fresh from his Florida meetings.”

Apropos of nothing, here’s an explosive read that digs into the history of big bombs to ask, “Was the first Soviet thermonuclear device really a step in the wrong direction?” It comes to us from Physics Today, and there’s a lot to unpack; so settle in with your favorite drink before you dive in, starting here.

President Trump met with the King of Jordan on Wednesday, and took some time for a few questions afterward. Much of the focus stuck to the recent alleged chemical weapons attack on Syrians in the northwestern governorate of Idlib—an attack the White House wasted little time this week pinning on the Assad regime.

Said Trump of the attack, “It crossed a lot of lines for me.” But in an uncharacteristic move for Trump, he told reporters his views on many issues are not inflexible—and that his perspective on the Assad regime is changing as a result. Trump even went so far as to call the attack “unacceptable.”

It was a puzzling moment for some inside the Pentagon, Buzzfeed’ Nancy Youssef reported. “He appeared to open the door to US action in response to the chemical attack, as had his ambassador to the United Nations, Nikki Haley, earlier in the day while chairing an emergency session of the Security Council.”

But the big questions, she writes, came sharp and fast, as “three defense officials told BuzzFeed News they cannot begin to craft a military response, if that is what Trump wants, without a clear understanding of what the president wants to see happen in Syria. Does he only want the Assad regime to stop using chemical weapons? Does he want regime change? Is he seeking a negotiated settlement? Or were Trump’s comments simply rhetoric?” More here.

From Defense One

US Missile Defenses Need Better Sensors, and Soon // CSIS’ Thomas Karako and Ian Williams: Gaps in coverage leave interceptors less-equipped to defeat the threats of tomorrow.

The St. Petersburg Bombing Brings Russia Face-to-Face with Transnational Terror // Control Risks’ Nabi Abdullaev, writing from Moscow: An immigrant from Central Asia kills 14, and forces Moscow to rethink its counterterrorism strategies.

Trump's Defense Hike Would Violate Budget Law, Congress' Analysts Confirm // Charles S. Clark: An April 3 report by the Congressional Research Service says the White House's draft 2018 request does not comply with the $549 billion cap.

Great-Power Rivalry May Turn Space War Real // The Atlantic’s Marina Koren: A rising drumbeat of remarks from Pentagon officials suggest they're worried about, and preparing for, warfare in orbit.

How Should the US Respond to Assad's Chemical Attacks? // Andrew Exum: Military intervention—even for the best moral reasons—always has unpredictable consequences.

Welcome to this Thursday edition of The D Brief by Ben Watson and Bradley Peniston. #OTD1862: the Battle of Shiloh begins. Got a message for us? Let us know by clicking this link to email:

Bannon exits the National Security Council: Washington Post: “President Trump on Wednesday removed controversial White House chief strategist Stephen K. Bannon from the National Security Council, part of a sweeping staff reshuffling that elevated military, intelligence and Cabinet officials to greater roles on the council and left Bannon less directly involved in shaping the administration’s day-to-day national security policy.” Who’s behind this? National Security Adviser Lt. Gen. H.R. McMaster, “who is increasingly asserting himself over the flow of national security information in the White House.” Read on, here.

Meanwhile, the Pentagon has a new public affairs leader. It’s Dana W. White, a communications consultant who “previously served as the Taiwan country director in the office of the under secretary of defense for Policy, a professional staff member on the Senate Armed Services Committee, and was an editorial writer for the Wall Street Journal,” a DoD release said.

Pilots’ boycott prompts Navy to ground its trainer jets. One day after Fox News reported “more than 100 U.S. Navy instructor pilots are refusing to fly in protest of what they say is the refusal of top brass to adequately address an urgent problem with training jets’ oxygen system,” Stars and Stripes reports that the Navy has grounded its T-45 Goshawk trainers. “Last Friday, we had roughly 40 percent of our flights canceled in the T-45 training commands in Meridian, Pensacola and Kingsville because of operational risk management concerns voiced by the instructor pilots,” Cmdr. Jeannie Groeneveld, a spokeswoman for Naval Air Forces, told Stripes.

For what it’s worth: “A flight instructor said the number was closer to 75 percent, because the Navy reduced the flight schedule knowing more than half the pilots would refuse to fly,” Fox reported.

What was/is going on: “Histotoxic hypoxia,” according to Fox News. That’s “the medical term associated with the disorientating disorder which can put pilots’ lives at risk, as well as those of civilians on the ground below. Two instructor pilots say the training jets are now averaging three incidents a week, as the Navy struggles to get to the bottom of the contamination...Last week, a student from training squadron VT-86 in Pensacola, Fla., had to be ‘dragged out’ of his jet because he became ‘incapacitated’ from the faulty oxygen system, according to two flight instructors.”

Said CDR Salamander of the broader dynamics: “When you can't breathe, nothing else matters. Take your stereotypical Naval Aviator; cocky, aggressive, devil-may-care, hour-hounding etc, etc ... now double it. That is your standard issue Navy Instructor Pilot. More often than not, the top pilots coming out of their first tour go on to some kind of billet as an Instructor Pilot at their FRS or a VT squadron. Keep that in mind.”

Said one pilot: “It can happen without warning. The system doesn’t detect contaminants.” Read on at Fox here, or at Stripes, here. Want some informed disgust over why the problem has taken so long (six years) to solve? CDR Salamander has you covered, here.

Also on Wednesday: An F-16C crashed near the capital, close to Joint Base Andrews. “The pilot had just taken off from Andrews along with three other F-16s to fly to a Pennsylvania training range for target practice, said Lt. Col. Mike Croker, commander of the 121st Fighter Squadron,” Stripes reported. “Several minutes after takeoff, the pilot realized the aircraft had experienced a mechanical issue, Croker said, without providing details...As the jet lost altitude, the pilot managed to turn the aircraft and jettison the fuel tanks before parachuting to safety, Croker said. The pilot ejected at a height between 2,000 and 3,000 feet.” More, here.

While we’re talking aircraft, the Marine Corps says the F/A-18s gotta go—and they want their F-35s sooner. Stripes, again: “The Hornets troubles have been well documented. The Marines lost five of the aircraft to crashes last year that killed three pilots — Maj. Richard Norton, 36, Capt. Jake Frederick, 32, and Capt. Jeff Kuss, 32. Nearly two dozen previously retired Hornets have been pulled from the Boneyard at Davis-Monthan Air Force Base in Arizona to be returned to the fleet.”

“The real answer is to get out of legacy Hornet business as fast as possible and transition to the F-35,” William E. Taylor, the assistant deputy commandant for aviation, told members of the House Armed Services’ Readiness subcommittee. More, here.

The Air Force is looking to Silicon Valley for “electronic cloaking technologies out of ‘Star Trek’ to shrink the profiles of aerial tankers on enemy radar,” Bloomberg reported Tuesday. Gen. Carlton Everhart, head of the Air Mobility Command has reportedly "spoken with technology companies, defense contractors, the Defense Advanced Research Project Agency and military service laboratories on an idea still in its earliest stage: Retrofitting tankers as old as the 1950s-era KC-135 and as new as Boeing Co.’s KC-46 with technology to alter the plane’s radar image, or waveform, so it appears to be ‘either in a different location in the air’ or ‘reduced or disappears altogether: Now you see me, now you don’t.’”

Quote of the day: “I’m piggybacking off industry,” Everhart said “If I put it in the military acquisition system, it would be 50 years before I get it out because of the regulations that we have.” Story, here.

Lastly today: The general who once led JSOC is on a campaign to save Sesame Street and the rest of its free TV educational pals on public broadcasting. That general would be Stanley McChrystal, writing in the op-ed pages of the New York Times Wednesday: “Public broadcasting makes our nation smarter, stronger and, yes, safer. It’s a small public investment that pays huge dividends for Americans. And it shouldn’t be pitted against spending more on improving our military.”

Said McChrystal: “This might seem like an unlikely position for me, a 34-year combat veteran. But it’s a view that has been shaped by my career leading brave men and women who thrive and win when they are both strong and smart. My experience has taught me that education, trusted institutions and civil discourse are the lifeblood of a great nation.”

His last word: “We need public media that acts as our largest classroom. We need broadcasting that treats us as citizens, not simply as consumers. We need a strong civil society where the connection between different people and groups is firm and vibrant, not brittle and divided. We need to defend against weaknesses within and enemies without, using the tools of civil society and hard power. We don’t have to pick one over the other.” Read on, here