washington syria bloody civil war slow-burning crisis eastern ukraine perennial diplomatic headaches north korean iranian nuclear program rex tillerson would face messy complex world chief u.s. diplomat brief descriptions problems land plate confirm u.s. senate 69th u.s. secretary state career spend exxon mobil corp rise chief executive tillerson inherit increasingly complex conflict syria rebel-held eastern portion aleppo verge fall syrian government force back russia iran shi ite militias lebanon iraq civil war well sixth year pit president bashar al-assad range rebel group many support outside power include unite state turkey gulf state saudi arabia qatar u.s. presidential campaign president-elect donald trump suggest top priority defeat islamic state militant group launch attack europe inspire mass-casualty attack unite state trump cooperate russia islamic state hold swath northeastern syria risk moderate rebel anger view abandonment unite state could gravitate toward militant islamist factions pose potential threat western interest moreover persistent bloodshed could unleash new destabilize wave refugees neighbor nations could attempt reach europe early refugee flow trigger backlash bolster far-right nationalist party europe cooperate russia would cause complications new u.s. administration one thing could extension align unite state iran lebanese proxy militia hezbollah iran use tie syria project force region trouble fact israeli officials eager see tehran extend influence next door iraq tillerson confront country struggle overcome deep ethnic sectarian divisions fight stamp islamic state insurgency help 6,000 u.s. support troop military advisers even iraqi security force prevail retake islamic state stronghold mosul country second-largest city extremist group expect revert wag guerrilla war shi ite-dominated central government could force trump tillerson decide whether unite state remain engage attempt counterbalance powerful influence neighbor shi ite-led iran play diminish role tillerson also may face renew drive iraq minority kurds train arm unite state fight islamic state independence oil-rich autonomous northern region move unite state oppose would raise threat conflict kurds baghdad could affect international petroleum market could also fuel demand kurds syria turkey great autonomy prospect turkish officials bitterly oppose north korea secretive authoritarian government conduct five nuclear test past decade four obama administration two year alone along unprecedented flurry ballistic missile test north test bring tight u.n. security council sanction include pass last month aim cut export earn quarter sign pyongyang young leader kim jong-un will restrain nuclear missile program reclusive impoverish north korea pose direct threat rich democratic south korea u.s. treaty ally technically remain state war pyongyang 1950-53 korean war end truce rather peace treaty north regularly threaten destroy south unite state maintain 28,000 troop south korea tripwire deter north korean attack north korea say september nuclear test master ability mount warhead ballistic missile develop effective intercontinental ballistic missile icbm could strike continental unite state trump harshly criticize agreement strike july 14 2015 iran six major power tehran agree restrain nuclear program exchange relief economic sanction many republicans particularly u.s. congress argue deal negotiate unite state britain china france germany russia tear trump critical agreement certain would actually abrogate one thing israeli officials quietly say prefer iran live deal restraints subject transparency measure totally free could allow race acquire nuclear weapons unite state blame agreement collapse would unlikely find much sympathy among european ally reimpose sanction tehran even deal hold tillerson may hand full try find ways counter iranian support syria assad hezbollah lebanon houthis militia fight saudi-backed government yemen tillerson may quickly find deal crisis trump make china give beijing angry response republican president-elect view washington necessarily stick long-standing position taiwan part one china upend four decades settle u.s. policy trump risk antagonize beijing could decide restrict cooperation unite state truly push seek subvert u.s. policy aim around world unite state vast array interest china third-largest destination export u.s. goods large source goods import u.s. market china also large holder u.s. government debt among areas two side work together obama administration maintain growth world economy rein iranian nuclear program seek combat climate change also disagreements north korea china maritime claim russia seize majority russian-speaking crimea ukraine march 16 2014 uprise topple ukraine pro-russian president ukraine unite state many governments western europe also say moscow arm encourage pro-russian separatists eastern ukraine covertly infiltrate russian military personnel conflict ukrainian force russian-backed separatists cost nearly 10,000 live since 2014. germany france try little success convince side implement peace deal agree minsk last year kremlin deny western charge stoke separatist movement aid rebel accuse ukraine perpetuate violence violate minsk deal unite state european union impose range economic sanction russia result annexation crimea action eastern ukraine trump say good relations russia could great benefit unite state suspicions nominee secretary state might incline ease western sanction russia one u.s. official say might deal do russian president vladimir putin will abide minsk peace deal agree something similar different name return sanction relief trump give specifics plan afghanistan nearly 10,000 u.s. troop remain 15 years islamist taliban topple u.s.-backed afghan force trump say however unite state stop carry nation build something seek decade half southwestern asian nation one important question face trump afghanistan former officials experts say many u.s. troop stay acknowledge afghan security remain precarious taliban force gain grind place president barack obama shelve plan cut u.s. presence almost half year end opt instead keep 8,400 troop end presidency january number provincial capitals pressure taliban afghan force suffer high casualty rat 5,500 kill first eight months 2016. syria north korea tillerson would inherit messy global situation