Yaƙin Duniya na biyu ya fara ne a ranar 3 ga Satumbar 1939 tare da ayyana yaƙi da Burtaniya da Faransa , kan Jamus na Nazi a matsayin martani ga mamayewar Poland da Jamus ta yi.Ƙungiyoyin Anglo-Faransa sun taimaka wa Poland .Yaƙin Phoney ya ƙare a cikin Afrilu 1940 tare da mamayewar Jamus na Denmark da Norway.Winston Churchill ya zama firaminista kuma shugaban gwamnatin hadin gwiwa a watan Mayun 1940. Kashin sauran kasashen Turai ya biyo baya - Belgium, Netherlands , Luxembourg da Faransa - tare da Sojojin Biritaniya wanda ya kai ga kwashe Dunkirk.Daga watan Yunin 1940, Biritaniya da daularta sun ci gaba da yakar Jamus ita kadai.Churchill ya tsunduma cikin masana'antu, masana kimiyya da injiniyoyi don ba da shawara da goyan bayan gwamnati da sojoji a cikin tuhumar ƙoƙarin yaƙi.Dakarun Sojin sama na Royal sun dakile shirin da Jamus ta yi na mamaye kasar Birtaniya, inda ta musanta cewa Luftwaffe ta fi karfin iska a yakin Biritaniya, da kuma gazawarta a karfin sojan ruwa.Bayan haka, yankunan Biritaniya sun sha fama da tashin bama-bamai a lokacin Blitz a karshen 1940 da farkon 1941. Sojojin ruwa na Royal sun nemi tarewa Jamus tare da kare jiragen ruwa na kasuwanci a yakin Atlantic.Sojojin sun kai farmaki a tekun Mediterrenean da Gabas ta Tsakiya, gami da yakin Arewa-Afurka da Gabashin-Afurka, da kuma yankin Balkan.Churchill ya amince da kawance da Tarayyar Soviet a watan Yuli kuma ya fara aika kayayyaki zuwa USSR.A cikin watan Disamba,daular Japan ta kai hari kan mallakar Birtaniyya da Amurka tare da kai hare-hare kusa-kusa da juna kan kudu maso gabashin Asiya da yankin Pacific ta Tsakiya ciki har da harin da jiragen ruwan Amurka a Pearl Harbor.Biritaniya da Amurka sun shelanta yaki akan Japan, inda suka bude yakin Pacific.An kafa Grand Alliance na Birtaniya, Amurka da Tarayyar Soviet kuma Birtaniya da Amurka sun amince da babbar dabara ta farko ta Turai don yaki.Birtaniya da ƙawayenta sun sha fama da mugun rauni a yaƙin Asiya da Fasifik a cikin watanni shida na farkon 1942.An sami nasara mai tsanani a cikin 1943 a yakin Arewacin Afirka, wanda Janar Bernard Montgomery ya jagoranta, da kuma yakin Italiya na gaba.Sojojin Birtaniyya sun taka rawa sosai wajen samar da bayanan sirri na Ultra, harin bama-bamai na Jamus, da saukar jiragen Normandy na Yuni 1944. 'Yantar da Turai ya biyo baya a ranar 8 ga Mayu 1945, wanda aka samu tare da Tarayyar Soviet, Amurka da sauran kasashen kawance. .Yaƙin Tekun Atlantika shi ne yaƙin neman zaɓe na soja mafi dadewa na yaƙin.A gidan wasan kwaikwayo na Kudu-maso-Gabas na Asiya, jiragen ruwan Gabas sun kai farmaki a tekun Indiya.Sojojin Burtaniya sun jagoranci yakin Burma na fatattakar Japan daga hannun turawan Ingila.Haɗin dakaru miliyan ɗaya a kololuwar sa, wanda aka zana da farko dagaBurtaniya Indiya , yaƙin neman zaɓe ya yi nasara a ƙarshe a tsakiyar 1945.Rundunar ruwan tekun Pasifik ta Biritaniya ta shiga yakin Okinawa da kuma harin da sojojin ruwa na karshe suka kaiwa Japan.Masana kimiyya na Burtaniya sun ba da gudummawa ga aikin Manhattan don kera makamin nukiliya.An sanar da mika wuya na Japan a ranar 15 ga Agusta 1945 kuma an sanya hannu kan 2 Satumba 1945.