Traditional allies of the United States are expressing concernabout the resignation of U.S. Defense Secretary Jim Mattis.President Donald Trump announced Thursdaythat Mattis will leave the job in February.The announcement came a day after Trump saidthat U.S. troops would leave Syria.The Defense Department opposed the troop withdrawal.For several years, U.S.-supported forces in the Middle Easthave made progress against the Islamic State (IS).Defeating IS forces was just one part of a plan Mattis had for Syria.He wanted to reduce U.S. troop numbers thereafter successful completion of the three-part plan.The president and Mattis disagreed on the issue.They also have different ideasabout support for America's traditional alliances.In his resignation letter, Mattis wrote that the United Statescannot protect its interests without "showing respect to (our) allies."The French Defense Minister Florence Parly spoke about the resignation.She said that French forces were in the Middle Eastas part of a coalition "led by the Americans."Trump's decision "changes that situation," she added.Britain's Minister of State for the Middle East, Alistair Burt,wrote that "If allies cannot be relied upon,others are sought to take their place."The social networking website Twitter published his comments.Former Swedish Prime Minister Carl Bildt tweeted the U.S. movewas a "morning of alarm in Europe."Defense Secretary Mattis is one of the most highly respectedmembers of Trump's foreign policy team.He helped lead the invasion of Iraq in 2003and later led the U.S. Central Command until he retired in 2013.In his resignation letter to Trump, Mattis wrote that the presidentshould have a Secretary of Defense who agreed with him on foreign policy.He added, "I believe it is right for me to step down from my position."In addition to Syria, the two men disagreed about other issues,such the president's decision to pull the United Statesout of the nuclear deal with Iran.They also have differences of opinion about Russian President Vladimir Putin.Trump has praised Putin's leadership skills.Mattis believed the Russian president was interfering with Western democracies.Russia has praised the U.S. decision to withdraw from Syria.In a separate move, Trump ordered the Defense Department to planto withdraw about 50 percent of the 14,000 U.S. troops in Afghanistan.That withdrawal will reportedly begin in January 2019.Reacting to Mattis' resignation, Senate Majority Leader Mitch McConnellsaid he was "distressed" that it resulted from"differences (about) America's global leadership."House Minority Leader Nancy Pelosi said she was "shaken"by the resignation and described it as "very serious for our country.""This is scary," tweeted Senator Mark Warner of Virginia.He is the top Democrat on the Senate Intelligence Committee.Trump's decision to leave Syria breakswith one of America's longtime allies, the Kurds.Turkish President Recep Tayyip Erdogan has threatened in recent daysto launch a military operation against U.S.-aided Kurdish militiamennow fighting in northeastern Syria.