President Donald Trump’s top national security officials briefed Congress on Wednesday on the administration’s strategy for North Korea — a briefing lawmakers said had an urgent tone and contained none of the bluster of the president’s tweets.Defense Secretary Jim Mattis, Joint Chiefs Chairman Gen. Joseph Dunford and Secretary of State Rex Tillerson laid out their plan for increasing pressure on North Korea and China to come to the negotiating table while also beefing up missile-defense systems to counter North Korea’s growing ballistic missile capabilities, according to lawmakers who attended the classified session.The briefing follows a series of new missile and nuclear tests by North Korea, including an underground detonation last weekend that the country claimed was a hydrogen bomb.