▁pl ann ers ▁had ▁concluded ▁that ▁it ▁would ▁be ▁imp ract ical ▁to ▁rep el ▁an ▁invasion ▁of ▁the ▁Philippines . ▁The ▁best ▁that ▁could ▁be ▁hoped ▁for ▁was ▁that ▁the ▁g arrison ▁could ▁hold ▁out ▁on ▁the ▁B ata an ▁pen ins ula ▁until ▁help ▁arrived . ▁In ▁the ▁ 1 9 2 0 s ▁it ▁was ▁estimated ▁that ▁they ▁could ▁do ▁so ▁for ▁about ▁ 6 0 ▁days . ▁By ▁the ▁ 1 9 3 0 s , ▁the ▁pl ann ers ▁had ▁become ▁decided ly ▁p ess im istic ▁in ▁view ▁of ▁the ▁increased ▁cap ability ▁of ▁aircraft , ▁and ▁by ▁ 1 9 3 6 ▁they ▁were ▁agreed ▁that ▁the ▁Philippines ▁should ▁be ▁written ▁off . ▁But ▁in ▁July ▁ 1 9 4 1 , ▁this ▁decision ▁was ▁ab rupt ly ▁revers ed , ▁and ▁it ▁became ▁the ▁policy ▁of ▁the ▁U . S . ▁government ▁to ▁defend ▁and ▁hold ▁the ▁Philippines . ▁This ▁was ▁based , ▁at ▁least ▁in ▁part , ▁in ▁the ▁belief ▁that ▁Bo eing ▁B - 1 7 ▁F lying ▁Fort ress ▁bom bers ▁could ▁d eter ▁or ▁defeat ▁an ▁inv ading ▁force . ▁ ▁So on ▁after ▁the ▁Japanese ▁invasion ▁of ▁the ▁Philippines ▁in ▁ 1 9 4 1 , ▁Mac Ar thur , ▁in ▁accord ance ▁with ▁the ▁pre - war ▁plan , ▁declared ▁Man ila ▁an ▁open ▁city , ▁and ▁ordered ▁his ▁forces ▁on ▁Lu zon ▁to ▁withdraw ▁to ▁B ata an . ▁The ▁Philipp ine ▁government , ▁the ▁High ▁Commission er