brussels u.s. defense secretary jim mattis tuesday play impact white house national security adviser michael flynn resignation might mattis debut trip week europe meet nato ally frankly impact obviously change head change message mattis tell reporters shortly land brussels praise nato alliance stress need adapt challenge pose russia president staff work say flynn resign late monday revelations discuss u.s. sanction russia russian ambassador unite state trump take office mislead vice president mike pence conversations flynn resignation occur hours report justice department warn white house weeks ago flynn could vulnerable blackmail contact russian ambassador sergei kislyak trump take power jan. 20. mattis take dim view prospect partnership moscow flynn presidential campaign trump since take office rule areas u.s. russian interest might align speak reporters tuesday mattis say north atlantic treaty organization need adapt challenge pose russia 2014 russia annexation ukraine crimea peninsula upend alliance assumptions cooperation moscow 2014 watershed year year many hop kind partnership russia show unavailing mattis say extol nato successful military alliance history get adapt make certain trans-atlantic bond remain strong say january senate confirmation hear mattis accuse russia try break nato alliance nl1n1f218b flynn contrast frequently raise eyebrows among washington foreign policy establishment try persuade trump warm u.s. relations russia one u.s. official say flynn departure couple russia aggression ukraine syria republican congressional opposition remove sanction russia remove trump ardent advocate take soft line toward russian president vladimir putin flynn leave may make significant course change less likely least time soon official say nl1n1fy1l9 another official say flynn departure might strengthen hand cabinet secretaries include mattis secretary state rex tillerson u.s. defense chief white house shakeup affect europe debut