WASHINGTON -- The situation in Syria is the "most complex security situation, fighting situation" he has experienced in his four decades of military service, Defense Secretary James N. Mattis said Monday.In a wide-ranging half-hour discussion with Pentagon reporters, Mattis discussed the situations in Syria and Afghanistan, as well as the Korean Peninsula.He said he was to discuss the way forward in Syria later in the day with the United Nations special envoy for Syria."I'm meeting with Staffan de Mitura [to] see where the Geneva process is and what we can do to assist," Mattis said. "And obviously, it's mostly a diplomatic effort in Geneva, but it has a military element, which says we don't allow ISIS to come back in the midst of all this."The United States, Mattis said, is working with all the countries in the region that are "engaged in trying to make stability a reality."Mattis met today with Secretary of State Mike Pompeo."What we don't want to do, now that we are on the cusp of winning on the battlefield in terms of taking down the physical caliphate, the geographic caliphate, we do not want to simply pull out before the diplomats have won the peace," he said. "So you win the fight, and then you win the peace."Nearly all the nations in the region are concerned about Iranian activities, he said.TALIBAN ON 'BACK FOOT'The Taliban and ISIS in Afghanistan are continuing to target civilians, Mattis said. "You can break them apart in terms of what organization they're part of, but their goal is to destabilize the elected government," he said.The Taliban were put on their "back foot" by strikes against their financial networks, the continued U.S. presence in Afghanistan, and Afghan President Ashraf Ghani saying his government was willing to negotiate with the Taliban, Mattis said."We anticipated that they would do their best to try to bring bombs right into Kabul," he said, pointing out the Taliban has targeted voting or voter registration locations, where crowds of innocent people gather."The Taliban realize the danger of the people being allowed to vote," he said, noting that citizens have risked their own safety to protest the Taliban in a number of demonstrations throughout the country.The United States, Mattis said, will continue to "fight and protect the innocent people."STRENGTHENED ALLIANCEMattis said he spent at least an hour on the phone April 28 with South Korean Defense Minister Song Young-moo. He said the call was part of the broader collaboration and constant consultation between the United States and South Korea, as developments unfold on the Korean Peninsula.Mattis said it is "pretty calm" on both the North and South Korean side, with no indicators or warnings of an increased military readiness in the North.The U.S.-South Korea alliance remains strong, he said, noting recent developments have strengthened the alliance even further. In a statement April 28, chief Pentagon spokesperson Dana W. White said the leaders discussed the results of the inter-Korea summit. Both leaders, the statement said, expressed their "serious commitment to a diplomatic resolution that achieves complete, verifiable and irreversible denuclearization of North Korea."