Overnight Defense: Washington waits for Trump response to Syria | Latest on chemical attack | National Guard troops head to border | Djibouti flights resume after crashes

View in your browser     THE TOPLINE: Washington on Monday debated how to respond to the new allegations of chemical weapons use by the Syrian regime. Over the weekend, horrifying pictures and videos emerged of civilians in the Damascus suburb of Douma dead and dying after an apparent chemical attack. At least 49 people have been reported dead and hundreds injured. Airstrikes hit a Syrian airbase Sunday night, but the Pentagon says it was not the U.S. military's doing. Syria and Russia claimed Israel carried out the strike, but Israel has not commented.   What might Trump do?: Almost exactly a year ago, the U.S. military launched 59 Tomahawk cruise missiles at a Syrian airbase in response to a sarin gas attack blamed on Syrian President Bashar Assad. Defense Secretary James Mattis wouldn't rule out airstrikes to respond to the latest attack. "I don't rule out anything right now," Mattis told reporters at the Pentagon prior to meeting with the emir of Qatar.   Lawmakers want action: Some U.S. lawmakers are calling for a response similar to last year's strike. "This happened about a year ago, where about 100 people were killed with a chemical attack. The president responded with a targeted strike against those Syrian military units that carried out the attack," Rep. Mac Thornberry (R-Texas), chairman of the House Armed Services Committee, said on Fox News. "I thought that made very good sense, and it seems to me we ought to consider doing it again with our allies." Other lawmakers are calling for even more. "This should be the last time a barrel bomb is dropped on innocent civilians by the Assad air force," Sen. Lindsey Graham (R-S.C.) said on "Fox and Friends." "We have the capability to destroy his air force, to ground his air fleet, and we should use that capability." Still others want a congressional vote before any military action is taken. "The use of chemical weapons absolutely requires a response from the United States," Sen. Mike Lee (R-Utah) said in a statement. "But if that response is going to include military force, the president of the United States should come to Congress and ask for authorization before military force is used."   When will we know?: President Trump promised a quick decision on how to respond to the chemical attack. Speaking at a Cabinet meeting at the White House, Trump told reporters he would make a decision "over the next 24 to 48 hours." "It was an atrocious attack. It was horrible," Trump told reporters during a Cabinet meeting at the White House. "This is about humanity and it can't be allowed to happen." Trump was scheduled to meet with his national security team later Monday.   Key background: The talk of a military strike on the Syrian regime comes after Trump talked about withdrawing U.S. troops from Syria altogether. The White House last week said U.S. troops would stay in Syria until the defeat of the Islamic State in Iraq and Syria (ISIS), but Trump gave the military a six-month deadline to complete the job in a private meeting with this national security team. The demand puts him at odds with his military advisors, who have argued U.S. troops need to stay to prevent ISIS's re-emergence, prevent Iranian influence from growing and keep the territory stable until Assad is removed from power. We explored the divide between Trump and his military advisors about Syria over the weekend here.   Tough words at the UN: U.S. Ambassador to the United Nations Nikki Haley on Monday blasted Moscow's support for Assad's regime, saying that Russian hands were "covered in blood" after the chemical attack.   Trump vs. McCain: Arizona Sen. John McCain (R) criticized Trump over the weekend, saying that his talk of withdrawing from Syria had emboldened Assad. On Monday, the White House hit back. "It is outrageous to say the president of the United States green-lit something as atrocious as the [chemical weapons attack]," White House press secretary Sarah Huckabee Sanders said.     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.   GUARD DEPLOYMENT UPDATE: President Trump's plan to deploy thousands of National Guard troops to the U.S.-Mexico border is underway. On Monday, Arizona Gov. Doug Ducey (R) announced the deployment of 225 guardsmen to support the mission, with an unspecified number of additional members expected to deploy Tuesday. "Just updated Arizona border sheriffs on today's deployment of National Guard. LATEST: 225 guard members being deployed today, additional members tomorrow," Arizona Gov. Doug Ducey (R) said in a tweet. "These troops will be helping our federal partners with any support role responsibilities that they need, and will be stationed in both the Tucson and Yuma sectors," he said in a second tweet.   What Mattis authorized: On Friday night, Defense Secretary James Mattis issued a memo that authorized up to 4,000 National Guard troops to support the Department of Homeland Security's border security mission. Mattis' memo stipulates that the guardsmen are not to perform law enforcement activities or interact with migrants or other individuals detained by DHS without Mattis' approval. The troops will be armed only if it's required for self-defense, the memo adds. "Together, the Departments of Homeland Security and Defense are committed to using every lever of power to support the men and women of law enforcement defending our nation's sovereignty and protecting the American people," Mattis and Homeland Security Secretary Kirstjen Nielsen said in a joint statement Friday. "We will continue to work with the governors to deploy the necessary resources until our nation's borders are secure."   Not the first time: Former President George W. Bush sent about 6,000 Guard troops to the border in 2006, while former President Obama sent about 1,200 guardsmen to the border in 2010. The deployments together were estimated to cost about $1.3 billion, according to the Government Accountability Office.   DJIBOUTI FLIGHTS BACK IN THE AIR: The U.S. military has resumed air operations in Djibouti following a stand down after two crashes last week. An AV-8B Harrier jet from the 26th Marine Expeditionary Unit crashed at Djibouti Ambouli International Airport last Tuesday, while a Marine CH-53 Super Stallion helicopter from the same unit sustained structural damage hours later during a landing at an approved exercise landing zone at Arta Beach, Djibouti.   Why it matters: Djibouti is a key hub for counterterrorism missions in the region. The nation is home to the only permanent U.S. military base on the continent. The base holds roughly 4,000 U.S. troops and functions as a launch point for operations in Somalia and Yemen.   AFGHANISTAN UPDATE: The U.S. military announced Monday that an airstrike last week killed a leader of Afghanistan's branch of the Islamic State in Iraq and Syria (ISIS). The April 5 strike in Faryab province killed Qari Hikmatullah and his bodyguard, according to the military. A news release described Hikmatullah as a senior commander of the branch, known as IS-K, and the main facilitator of IS-K fighters into northern Afghanistan.   NORTH KOREA UPDATE: President Trump his planned nuclear summit with Korean leader Kim Jong Un could take place in May or June. Officials had previously floated meetings by May, raising the possibility that the timeline for talks could slip. Takeaway: Trump's comments suggest he is still committed to meeting with Kim despite the obstacles ahead.   ON TAP FOR TOMORROW: The Senate Armed Services Committee will hold a hearing on Transportation Command's fiscal 2019 budget request at 9:30 a.m. at the Dirksen Senate Office Building, room G-50. https://bit.ly/2Eqv6c8 A Senate Foreign Relations Committee subpanel will hold a hearing on the Summit of the Americas at 10 a.m. at Dirksen 419. https://bit.ly/2EucfNK    ICYMI: -- The Hill: US conducts successful test to integrate missile-defense systems -- The Hill: Army: Two died in helicopter crash in Kentucky -- The Hill: Opinion: North Korea dangles 'denuclearization' like a hunter setting a trap -- The Hill: Opinion: Apparent Syrian chemical attack must not change Trump foreign policy -- The Hill: Opinion: After reported chemical attack in Syria, Trump's 'loose cannon' instincts are his best asset -- Military Times: The death toll for rising aviation accidents: 133 troops killed in five years -- Associated Press: Military child abuse case raises complex sentencing issues -- Lubbock Avalanche-Journal: Levelland native talks about nomination as head of VA     Please send tips and comments to Rebecca Kheel, rkheel@thehill.com, and Ellen Mitchell, emitchell@thehill.com. 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