Bayan kawo karshen yakin Franco-Thai, gwamnatin kasar Thailand ta ayyana matsayinta na tsaka mai wuya.Lokacin daJapanawa suka mamaye Tailandia a ranar 8 ga Disamba 1941, 'yan sa'o'i kadan bayan harin da aka kai a Pearl Harbor , Japan ta bukaci 'yancin tura sojoji a fadin Thailand zuwa iyakar Malayan .Phibun ya karɓi buƙatun Japan bayan ɗan gajeren juriya.Gwamnati ta kyautata alaka da kasar Japan ta hanyar sanya hannu kan wata kawancen soji a watan Disamba na shekara ta 1941. Sojojin Japan sun yi amfani da kasar a matsayin sansanin da suka mamaye Burma da Malaya.^[63] .^[64] A wata mai zuwa, Phibun ya shelanta yaki a kan Biritaniya da Amurka .Afirka ta Kudu da New Zealand sun shelanta yaki a Thailand a rana guda.Ostiraliya ta biyo baya ba da daɗewa ba.^{[65 <>]} An kori duk waɗanda ke adawa da kawancen Japan daga gwamnatinsa.An nada Pridi Phanomyong a matsayin mai rikon kwarya ga Sarki Ananda Mahidol da ba ya nan, yayin da Direk Jayanama, fitaccen ministan harkokin wajen kasar wanda ya ba da shawarar ci gaba da adawa da Japanawa, aka tura Tokyo a matsayin jakada.Amurka ta dauki Thailand a matsayin yar tsana ta Japan kuma ta ki ayyana yaki.Lokacin da ƙawayen suka yi nasara, Amurka ta toshe ƙoƙarin Birtaniyya na sanya zaman lafiya mai ɗorewa.^[66]Thais da Japan sun yarda cewa jihar Shan da Kayah za su kasance karkashin ikon Thai.A ranar 10 ga watan Mayun shekarar 1942 ne sojojin Phayap na kasar Thailand suka shiga jihar Shan ta gabashin kasar Burma, sojojin yankin Burma na kasar Thailand sun shiga jihar Kayah da wasu yankunan tsakiyar kasar ta Burma.Sojojin kasa uku na kasar Thailand da na sojan doki guda daya, wadanda ke karkashin jagorancin kungiyoyin leken asiri masu sulke da kuma goyon bayan sojojin sama, sun shiga runduna ta 93 ta kasar Sin da ke ja da baya.An kama Kengtung, babban makasudin, a ranar 27 ga Mayu.Sabbin hare-hare a watan Yuni da Nuwamba sun sa Sinawa sun koma Yunnan.^[67] Yankin da ke ɗauke da jihohin Shan da Jihar Kayah ta kasance ƙasar Thailand a cikin 1942. Za a mayar da su Burma a 1945.Seri Thai (Free Thai Movement) wani yunkuri ne na juriya na karkashin kasa akan Japan wanda Seni Pramoj, jakadan Thai a Washington ya kafa.An jagorance shi daga cikin Thailand daga ofishin regent Pridi, yana gudanar da aikin kyauta, sau da yawa tare da goyon baya daga membobin gidan sarauta kamar Yarima Chula Chakrabongse, da membobin gwamnati.Yayin da Japan ta kusa shan kashi kuma adawar Japan na adawa da Seri Thai ke ci gaba da samun karfi, Majalisar Dokoki ta kasa ta kori Phibun.Mulkinsa na shekaru shida a matsayin babban kwamandan sojoji ya kare.A wani bangare na murabus din nasa ya sa wasu manyan tsare-tsarensa guda biyu sun tabarbare.Daya shi ne mayar da babban birnin kasar daga Bangkok zuwa wani wuri mai nisa a cikin daji kusa da Phetchabun a arewa ta tsakiyar Thailand.Dayan kuma shi ne ya gina "birnin Buddha" kusa da Saraburi.An sanar da shi a lokacin mawuyacin halin tattalin arziki, waɗannan ra'ayoyin sun sa jami'an gwamnati da yawa suka ƙi shi.^[68]A karshen yakin, an gurfanar da Phibun a gaban shari'a a kan dagewar Allied kan zargin aikata laifukan yaki, musamman na hada kai da ikon Axis.Sai dai kuma an wanke shi ne sakamakon matsin lamba da jama'a suka yi masa.Ra'ayin jama'a har yanzu yana da kyau ga Phibun, saboda ana tunanin ya yi iya ƙoƙarinsa don kare muradun Thai, musamman yin amfani da ƙawance da Japan don tallafawa faɗaɗa yankin Thai a Malaya da Burma.^[69]