World War II

====War breaks out in Europe (1939–1940)====
{{Main|European theatre of World War II}}
[[File:Germans at Polish Border (1939-09-01).jpg|thumb|Soldiers of the German ''[[Wehrmacht]]'' tearing down the border crossing into [[Second Polish Republic|Poland]], 1 September 1939]]

On 1 September 1939, Germany [[Invasion of Poland|invaded Poland]] after [[Operation Himmler|having staged]] several [[Gleiwitz incident|false flag border incidents]] as a pretext to initiate the invasion.{{sfn|Evans|2008|pp=1–2}} The first German attack of the war came against the [[Battle of Westerplatte|Polish defenses at Westerplatte]].<ref name="Zabecki2015">{{cite book|url=https://books.google.com/books?id=Mq_lCAAAQBAJ&pg=PT1663|title=World War II in Europe: An Encyclopedia|author=David T. Zabecki|date=2015|publisher=Routledge|isbn=978-1-135-81242-3|page=1663|quote=The earliest fighting started at 0445 hours when marines from the battleship Schleswig-Holstein attempted to storm a small Polish fort in Danzig, the Westerplate|access-date=17 June 2019|archive-date=7 March 2023|archive-url=https://web.archive.org/web/20230307201256/https://books.google.com/books?id=Mq_lCAAAQBAJ&pg=PT1663|url-status=live}}</ref> The United Kingdom responded with an ultimatum for Germany to cease military operations, and on 3 September, after the ultimatum was ignored, Britain and France declared war on Germany,<ref>[[United Kingdom declaration of war on Germany (1939)|The UK declared war on Germany]] at 11 am. [[French declaration of war on Germany (1939)|France followed 6 hours later]] at 5 pm.</ref> followed by [[Australia]], [[Dominion of New Zealand|New Zealand]], [[Union of South Africa|South Africa]], and [[Canada]]. During the [[Phoney War]] period, the alliance provided no direct military support to Poland, outside of a [[Saar Offensive|cautious French probe into the Saarland]].<ref name="Keegan 1997 35">{{Harvnb|Keegan|1997|p=35}}.<br />{{Harvnb|Cienciala|2010|p=128}}, observes that, while it is true that Poland was far away, making it difficult for the French and British to provide support, "[f]ew Western historians of World War&nbsp;II&nbsp;... know that the British had committed to bomb Germany if it attacked Poland, but did not do so except for one raid on the base of Wilhelmshaven. The French, who committed to attacking Germany in the west, had no intention of doing so."</ref> The Western Allies also began a [[Blockade of Germany (1939–1945)|naval blockade of Germany]], which aimed to damage the country's economy and war effort.<ref>{{Harvnb|Beevor|2012|p=32}}; {{Harvnb|Dear|Foot|2001|pp=248–249}}; {{Harvnb|Roskill|1954|p=64}}.</ref> Germany responded by ordering [[Submarine warfare#World War II|U-boat warfare]] against Allied merchant and warships, which would later escalate into the [[Battle of the Atlantic]].<ref>{{Cite web |title=Battle of the Atlantic |url=https://www.history.co.uk/history-of-ww2/battle-of-the-atlantic |access-date=11 July 2022 |website=Sky HISTORY TV channel |language=en |archive-date=20 May 2022 |archive-url=https://web.archive.org/web/20220520073745/https://www.history.co.uk/history-of-ww2/battle-of-the-atlantic |url-status=live }}</ref>

On 8 September, German troops reached the suburbs of [[Warsaw]]. The Polish [[Battle of the Bzura|counter-offensive]] to the west halted the German advance for several days, but it was outflanked and encircled by the ''[[Wehrmacht]]''. Remnants of the Polish army broke through to [[Siege of Warsaw (1939)|besieged Warsaw]]. On 17 September 1939, two days after signing a [[Battles of Khalkhin Gol|cease-fire with Japan]], the [[Soviet invasion of Poland|Soviet Union invaded Poland]]{{sfn|Zaloga|2002|pp=80, 83}} under the supposed pretext that the Polish state had ceased to exist.<ref>{{Cite journal |jstor = 2195670|title = A Case Study in the Soviet Use of International Law: Eastern Poland in 1939|journal = The American Journal of International Law|volume = 52|issue = 1|pages = 69–84|last1 = Ginsburgs|first1 = George|year = 1958|doi = 10.2307/2195670|s2cid = 146904066}}</ref> On 27 September, the Warsaw garrison surrendered to the Germans, and [[Independent Operational Group Polesie|the last large operational unit of the Polish Army]] [[Battle of Kock (1939)|surrendered on 6{{nbsp}}October]]. Despite the military defeat, Poland never surrendered; instead, it formed the [[Polish government-in-exile]] and a [[Polish Underground State|clandestine state apparatus remained]] in occupied Poland.{{sfn|Hempel|2005|p=24}} A significant part of Polish military personnel [[Romanian Bridgehead|evacuated to Romania]] and Latvia; many of them later [[Military history of Poland during World War II|fought against the Axis]] in other theatres of the war.{{sfn|Zaloga|2002|pp=88–89}}

Germany [[Polish areas annexed by Nazi Germany|annexed western]] Poland and [[General Government|occupied central Poland]]; the Soviet Union [[Territories of Poland annexed by the Soviet Union|annexed eastern Poland]]; small shares of Polish territory were transferred to [[Territorial evolution of Poland#World War II|Lithuania]] and [[Slovak invasion of Poland|Slovakia]]. On 6 October, Hitler made a public peace overture to the United Kingdom and France but said that the future of Poland was to be determined exclusively by Germany and the Soviet Union. The proposal was rejected<ref name=ibiblio1939 /> and Hitler ordered an immediate offensive against France,<ref>Nuremberg Documents C-62/GB86, a directive from Hitler in October 1939 which concludes: "The attack [on France] is to be launched this Autumn if conditions are at all possible."</ref> which was postponed until the spring of 1940 due to bad weather.{{sfn|Liddell Hart|1977|pp=39–40}}{{sfn|Bullock|1990|loc=pp. 563–64, 566, 568–69, 574–75 (1983 ed.)}}<ref>Blitzkrieg: From the Rise of Hitler to the Fall of Dunkirk, L Deighton, Jonathan Cape, 1993, pp. 186–87. Deighton states that "the offensive was postponed twenty-nine times before it finally took place."</ref>

[[File:Karelian Isthmus 13 March 1940.png|thumb|left|[[Mannerheim Line]] and [[Karelian Isthmus]] on the last day of the [[Winter War]], 13 March 1940]]
After the outbreak of war in Poland, Stalin threatened [[Estonia]], [[Latvia]], and [[Lithuania]] with military invasion, forcing the three [[Baltic states|Baltic countries]] to sign [[Background of the occupation of the Baltic states#Soviet ultimatums and occupation|pacts]] allowing the creation of Soviet military bases in these countries; in October 1939, significant Soviet military contingents were moved there.{{sfn|Smith|Pabriks|Purs|Lane|2002|p=24}}{{sfn|Bilinsky|1999|p=9}}{{sfn|Murray|Millett|2001|pp=55–56}} [[Finland]] refused to sign a similar pact and rejected ceding part of its territory to the Soviet Union. [[Winter War#Soviet invasion|The Soviet Union invaded Finland]] in November 1939,{{sfn|Spring|1986|pp=207–226}} and was subsequently expelled from the [[League of Nations]] for this crime of aggression.<ref>Carl van Dyke. ''The Soviet Invasion of Finland''. Frank Cass Publishers, Portland, OR. {{ISBN|978-0-7146-4753-1}}, p. 71.</ref> Despite overwhelming numerical superiority, Soviet military success during the [[Winter War]] was modest,<ref>{{cite web|url=https://www.warhistoryonline.com/war-articles/winter-war-finland.html|title=The Winter War – When the Finns Humiliated the Russians|first=Ivano|last=Massari|publisher=War History Online|date=18 August 2015|access-date=19 December 2021|archive-date=19 December 2021|archive-url=https://web.archive.org/web/20211219185618/https://www.warhistoryonline.com/war-articles/winter-war-finland.html|url-status=live}}</ref> and the Finno-Soviet war ended in March 1940 with [[Moscow Peace Treaty|some Finnish concessions of territory]].{{sfn|Hanhimäki|1997|p=12}}

In June 1940, the Soviet Union [[Soviet occupation of the Baltic states (1940)|occupied]] the entire territories of Estonia, Latvia and Lithuania,{{sfn|Bilinsky|1999|p=9}} as well as the Romanian regions of [[Soviet occupation of Bessarabia and Northern Bukovina|Bessarabia, Northern Bukovina, and the Hertsa region]]. In August 1940, Hitler imposed the [[Second Vienna Award]] on Romania which led to the transfer of [[Northern Transylvania]] to Hungary.{{sfn|Dear|Foot|2001|pp=745, 975}} In September 1940, Bulgaria demanded [[Southern Dobruja]] from Romania with German and Italian support, leading to the [[Treaty of Craiova]].<ref name="Haynes-2000">{{cite book|url=https://books.google.com/books?id=b_I-AQAAIAAJ|title=Romanian policy towards Germany, 1936–40|first=Rebecca|last=Haynes|publisher=[[Palgrave Macmillan]]|page=205|year=2000|isbn=978-0-312-23260-3|access-date=3 February 2022|archive-date=7 March 2023|archive-url=https://web.archive.org/web/20230307201243/https://books.google.com/books?id=b_I-AQAAIAAJ|url-status=live}}</ref> The loss of one-third of Romania's 1939 territory caused a coup against King Carol II, turning Romania into a fascist dictatorship under Marshal [[Ion Antonescu]], with a course set towards the Axis in the hopes of a German guarantee.<ref>Deletant, pp. 48–51, 66; Griffin (1993), p. 126; Ornea, pp. 325–327</ref> Meanwhile, German-Soviet political relations and economic co-operation{{sfn|Ferguson|2006|pp=367, 376, 379, 417}}{{sfn|Snyder|2010|pp=118ff}} gradually stalled,{{sfn|Koch|1983|pp=912–914, 917–920}}{{sfn|Roberts|2006|p=56}} and both states began preparations for war.{{sfn|Roberts|2006|p=59}}