Rubuta ci gaba ga wannan sakin layi - Truman ya tambayi 'yan majalisar Roosevelt su ci gaba da kasancewa a matsayinsu, yana gaya musu cewa yana bude ga shawarwarinsu. Ya nanata muhimmiyar ƙa'ida ta sarautarsa: shi ne zai tsai da shawara, kuma za su goyi bayansa. A lokacin Yaƙin Duniya na II, tare da mamayewar Japan da ke gabatowa, ya amince da jadawalin jefa bama-bamai biyu na atomic don kauce wa mamayewar tsada na ƙasar Japan. Ƙididdigar sojoji don mamayewar Japan ita ce cewa zai iya ɗaukar shekara guda kuma ya haifar da asarar rayukan Amurka 250,000 zuwa 500,000. Amurka ta jefa bama-bamai a Hiroshima a ranar 6 ga Agusta, da Nagasaki kwana uku bayan haka, ta bar mutane 105,000 da suka mutu. Tarayyar Soviet ta yi wa Japan yaƙi a ranar 9 ga watan Agusta kuma ta mamaye Manchuria
Ka yi haƙuri. Japan ta amince ta mika wuya a rana mai zuwa. Truman ya ce kai wa Japan hari da bama-baman nukiliya ya ceci rayuka da yawa a bangarorin biyu. Da ƙarshen Yaƙin Duniya na II, Truman ya aiwatar da Shirin Marshall. Baya ga zaben fidda gwani da yakin neman zabe a shekarar 1948, Truman ya yi mu'amala da toshewar Berlin, wanda ake ganin shi ne babban rikicin diflomasiyya na farko na Yakin Cacar Baki. A lokacin shugabancinsa, ra'ayoyin amincewa da Truman ya ragu sosai daga 80% a farkon 1945 zuwa 30% a farkon 1947. Zaɓen tsakiyar shekara ta 1946 ya firgita Truman lokacin da 'yan Republican suka sami ikon kula da gidaje biyu na Majalisa a karon farko tun daga shekarun 1920. A shekara ta 1947, Truman ya gaya wa Sakataren Tsaronsa James Forrestal cewa, ban da lada na sabis, bai sami gamsuwa da zama Shugaban kasa ba.