Someone’s missiles hit Syrian base; 500 troops to the Mexico border; John Bolton; The trouble with ‘denuclearization’; China put jammers on the Spratlys; And a bit more.

Whose missiles struck Syria overnight? Was it Israel? Pentagon officials spent Sunday denying those were U.S. military missiles seen flying over Lebanon to strike a Syrian air base, but it’s unclear who is responsible for the attack — yet.

Expectations of a U.S. strike on Syria increased after President Trump fingered Russian President Vladimir Putin in a tweet about the Saturday attacks:

• "Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price to pay..."

That was followed by: Trump and French President Emmanuel Macron put out a joint statement Sunday evening, vowing a “strong” response to the alleged attack. Macron and Trump are speaking regularly laterly. Recall that Macron — standing beside Putin — warned last May further use of chemical weapons in Syria was his “red line.”

Monday morning Mattis: The SecDef fingered Russia for the attacks, too, saying they were “guarantor of removing all the chemical weapons...we are going to address this issue” with allies. A Pentagon pool reporter asked, at a morning photo op with visiting Qataris, “Can you rule out taking actions, launching airstrikes against Assad...?” Mattis: “I don't rule out anything right now.”

So...Israel? The answer could emerge at an emergency meeting of the UN Security Council later today, if not sooner.

Just now catching up? Here’s what you missed: “Syria and its main ally Russia blamed Israel for carrying out an attack on a Syrian air base near Homs on Monday which followed reports of a poison gas attack by President Bashar al-Assad’s forces on a rebel-held town” on Saturday, Reuters reports this morning from neighboring Jordan.

Unusual air activity over Lebanon Sunday evening appears to be evident in videos posted to social media, here and here.

Targeted overnight: "Syria’s strategic T4/Tiyas airbase in Homs...using jets that flew over southern Lebanon," according to the Middle East Institute's Charles Lister.

Russia claims: “Israel fired 8 missiles; 5 were destroyed by Syrian defence systems but remaining three struck western part of T-4 airbase,” Reuters writes. “At least 14 people were killed including some fighters of various nationalities, a reference to Iranian-backed Shi’ite militia members, mostly from Iraq, Lebanon and Iran fighting alongside the Syrian army.”

Fast-moving developments: “In the span of five minutes,” the Organisation for the Prohibition of Chemical Weapons said it would investigate the Saturday attack in Douma. And moments later, Russia’s foreign minister said Moscow “already concluded there was no trace of chemical use,” Agence France-Presse’s Syria-watcher Maya Gebeily reports.

FWIW: The Israeli Air Force last struck the T4 base in February, after they had an F-16 shot down by Syrian air defense units on February 10, Lister noted.

What the strike on Syria’s T4 air base means, according to recently retired Israel Defense Forces spox, Lt. Col. Peter Lerner:

1. “Israel (according to Russia) continues to safeguard its core interests, despite risks.
2. Russia is no longer going to give Israel a free pass to leave random explosions in Syria unattributed.
3. The ball is now with Iran, Syria & Hezbollah.”

The easier question this morning: Who dropped what appear to have been chemical weapons on Syrian rebels near Damascus on Saturday?

Grisly account from paramedic on scene: “We gave them whatever we had, which wasn’t much, just four oxygen generators and atropine so they could breathe,” one first responder told The Guardian. “Most of them were going to die,” he continued. “I’ve been working in this hospital for five years and those last two days, I haven’t seen anything like it.” More here.

New this morning: Reuters published a lengthy investigation into “How Russian military support is secretly airlifted to Syria's Assad,” and you can dive in, here.

Is now still a good time to pull U.S. troops out of Syria? Here’s a thought: “For folks saying fewer troops in Syria ends U.S. influence in the Middle East, there's 52,000 U.S. troops in the region, outside Syria,” the Chatham House’s Micah Zenko noted this weekend.

What’s more, he warns, “If the U.S. cannot ‘shape’ outcomes in the Middle East with just 52,000 troops, then that's a good starting point for debating U.S. strategy in the region.”

From Defense One

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The Threat Within NATO // Rob Berschinski: An alliance built on democratic ideals is seeing the rise of strongmen in its midst.

Trumpism: Speak Loudly and Carry a Big Stick // Uri Friedman: 'It's entirely possible that the blowup is just yet to come,' says one observer of the president's foreign policy.

Welcome to this Monday edition of The D Brief by Ben Watson and Kevin Baron. And if you find this useful, consider forwarding it to a friend or colleague. They can subscribe here for free. On this day in 2013, the Islamic State of Iraq changed its name to the Islamic State of Iraq and the Levant, or the Islamic State of Iraq and al-Sham.

John Bolton Day. Trump’s new national security advisor starts work today with a new batch of U.S. National Guard troops ordered to the U.S.-Mexico border late Friday.
According to Houston’s ABC13 news, reporting early this morning, “Nearly 500 members will be sent to five different sectors along the U.S.-Mexico border today.”
What the troops won’t do: “perform law enforcement activities or interact with migrants or other individuals detained by DHS without approval from Mattis,” the Defense Department said in a release. As well, “Arming will be limited to circumstances that might require self-defense.”
In video: “Fox & Friends” spoke with a few Guard officials about what they’re up to, here.
As many as 4,000 troops have been authorized to support the Department of Homeland Security’s “southern border security mission while under the command and control of their respective governors through September 30, 2018,” according to a memo (PDF) from Defense Secretary Mattis.
The more you know. Texas has a “Military Department,” and it tweeted out a photo of officials talking about what they’re going to do, here.

North Korean officials told the U.S. they are prepared to discuss “denuclearization,” two unnamed U.S. officials told Reuters on Sunday.
What Reuters is hearing: “The communications, still at a preliminary stage, have involved State Department officials talking to North Korea apparently through its United Nations mission, and intelligence officers from both sides using a separate backchannel.”
The difference: “Until now, the United States had relied mostly on ally South Korea’s assurance of Kim’s intentions.”
Josh Rogin says nah, guys: “They have always said publicly they are prepared to discuss the denuclearization of the Korean Peninsula. This is not new. The issue is on what terms. No sign North Korea has changed its stance on that.”
The reax from South Korea: “We are aware contact between North Korea and the United States is going well. We don’t know, however, up to what extent information is being shared between the two.”
About that word — “denuclearization” — Reuters writes, “Questions remain about how North Korea would define denuclearization, which Washington sees as Pyongyang abandoning its nuclear weapons program.”
The rub: “North Korea has said over the years that it could consider giving up its nuclear arsenal if the United States removed its troops from South Korea and withdrew its so-called nuclear umbrella of deterrence from South Korea and Japan.”

The next big question: Is the world safer with a nuclear North Korea, or a denuclearized South Korea? asked Jonathan Cheng of The Wall Street Journal, speaking to NPR’s Steve Inskeep today on Morning Edition.
Notes MIT’s Vipin Narang: “Trump thinks he’s going to a summit to talk about taking away Kim’s nuclear weapons. Kim thinks he’s going to a summit to talk about how quickly the U.S. is going to get the hell off ‘his’ peninsula. Definitely gonna go great.” More from than angle over at the Washington Post, here.
Who’s excited about the inter-Korean summit? Seoul’s Yonhap News agency, The Diplomat’s Ankit Panda noted on Twitter this weekend.  
From the region: U.S. officials say China just put down some new military jamming equipment on the contested Spratly Islands in the South China Sea, the Wall Street Journal reports (paywall alert).

Top American tech companies to Trump: Take on China with allies, not tariffs. That opinion came this morning via the Information Technology Industry Council, "which counts information technology hardware, software, services and social media companies from Apple Inc (AAPL.O) to Twitter Inc (TWTR.N)," Reuters reports as the White House is threatening to impose an additional $100 billion in additional tariffs aimed at Chinese imports.
From the group: “Instead of tariffs, we strongly encourage the administration to build an international coalition that can challenge China at the World Trade Organization and beyond... Numerous countries share the United States’ concerns about China and its unfair trade practices. The United States is uniquely well-situated to lead that coalition.” More here.

U.S. military flight accidents up 40 percent since 2013.  “Accidents involving all of the military’s manned fighter, bomber, helicopter and cargo warplanes rose nearly 40 percent from fiscal years 2013 to 2017,” Military Times reported Sunday after some FOIA work — and a lot of presentation effort to display the reams of data they obtained. (Well done, friends.)
What’s going on? “The rise is tied, in part, to the massive congressional budget cuts of 2013,” Mil Times’ Tara Copp writes. “Since then, it’s been intensified by non-stop deployments of warplanes and their crews, an exodus of maintenance personnel and deep cuts to pilots’ flight-training hours.” Check out the database for yourself — searchable by aircraft type, year, mishap class and location — here.

Related: Djibouti flight ops for the U.S.  military are back online, CNN’s Ryan Browne reported Sunday citing a U.S. defense official. (A non-fatal crash there on April 3 had temporarily grounded flights.)
From the region: Somali officials just seized nearly $10 million in cash from a UAE plane in Mogadishu, Voice of America reported Sunday.

And lastly this morning: What you could do with $10 million — sleep in a luxury hotel 200 miles above the Earth. Bloomberg has the story of the “Aurora Station,” from Houston-based Orion Span Inc. (Images, here.)
The company’s plan: “launch the modular station in late 2021 and welcome its first guests the following year, with two crew members accompanying each excursion.”
Oh, the views: “The platform would orbit 200 miles above Earth, offering six guests 384 sunrises and sunsets as they race around the planet for 12 days at incredibly high speeds.”
Ouch, the cost: $9.5 million per person for a 12-day stay, which comes to about $791,666 a night, Bloomberg notes. Much more to that story, here.