washington/tokyo president donald trump defense secretary expect underscore u.s. security commitments key ally south korea japan debut trip asia week concern mount north korea missile program tensions china trip first retire marine general jam mattis since become trump pentagon chief also first foreign trip trump cabinet secretaries officials say fact mattis first head asia oppose perhaps visit troop iraq afghanistan mean reaffirm tie two asian ally host nearly 80,000 american troop importance region overall u.s. reaffirmation could critical trump appear question cost u.s. alliances election campaign also jolt region pull washington asia-pacific trade deal japan champion reassurance message say one trump administration official speak condition anonymity people concern campaign then-candidate now-president trump skeptical alliances somehow go retreat traditional leadership role region trump speak leaders japan south korea recent days host japanese prime minister shinzo abe washington feb. 10. mattis leave unite state feb. head first seoul continue tokyo feb. 3. trump single south korea japan campaign trail suggest benefit u.s. security umbrella without share enough cost one 2016 television interview trump say 28,500 u.s. troop deploy south korea get practically nothing compare cost mattis confirmation hear appear play remark note long history u.s. presidents even defense secretaries call ally pay fair share defense cost visit region come amid concern north korea may ready test new ballistic missile could early challenge trump administration speak south korean defense minister han min-koo ahead trip mattis reaffirm u.s. commitment defend country provide extend deterrence use full range u.s. capabilities analysts expect mattis seek update south korea early move host terminal high altitude area defense thaad system place sometime 2017 would defend north korea nuclear ballistic capabilities still south korean military official play expectations big announcements trip say mattis first visit would likely ice-breaking session countries tokyo mattis meet defense minister tomomi inada repeatedly say japan bear fair share cost u.s. troop station stress alliance good nations japan defense spend remain around percent gdp far behind china lock dispute japan group east china sea islets 220 km 140 miles northeast taiwan know senkakus tokyo diaoyus beijing trip also come amid grow concern china military move south china sea tension beijing escalate last week trump white house vow defend international territories china respond say irrefutable sovereignty dispute islands strategic waterway u.s. military people say consider pace china military build-up anti-ship missiles fighters worry japan capabilities say senior japanese defense ministry official speak condition anonymity trump 's defense chief head asia eye china north korea threat