washington u.s. president donald trump defense secretary plan visit japan south korea next week choose two close u.s. ally debut trip abroad pentagon chief u.s. official say tuesday speak condition anonymity jam mattis retire marine general swear pentagon top job shortly trump inauguration friday quick departure asia could see nod importance new u.s. administration place security tie trip would closely follow trump withdrawal 12-nation trans-pacific partnership tpp trade pact fulfil campaign pledge disappoint many key u.s. ally asia japanese prime minister shinzo abe example tout tpp part counter-weight rise china detail immediately available mattis propose itinerary still mattis confirmation hear month describe pacific theater priority analysts expect new u.s. military spend trump administration would strengthen america military presence asia time top u.s. concern region north korea nuclear weapons missile program china military move south china sea tension beijing escalate week trump white house vow defend international territories strategic waterway china respond say irrefutable sovereignty dispute islands mattis senate testimony also voice concern north korea describe pyongyang activities serious threat require u.s. attention 28,500 u.s. troop base south korea help defend country north korea technically remain state war south since 1950-53 korean conflict south korea unite state say upcoming deployment terminal high altitude area defense thaad system good help protect seoul north korea nuclear ballistic capabilities china say thaad powerful radar could penetrate territory prompt call south korean opposition leaders delay cancel deployment trump 's defense chief eye debut trip japan south korea next week