The Polish Civil War was an armed conflict in central Europe (primarily Poland), fought between September 13th, 1928 and May 20th, 1929. Ostensibly, it was a war between Polish governmental loyalists and Marxist insurgents (united as the Communist Party of Poland), but was characterised by the intervention of Germany on September 27th, 1928, in support of the Polish government. Whilst the loyalist cause ultimately prevailed, the major outcome of the war would be Germany's annexation of the Danzig Corridor, ridding Poland of a Baltic coast.
The origins of the conflict lie in the Soviet Union's long-standing support of the Communist Party of Poland (KPP), which, by the latter half of the 1920s, had transformed the organization into a very competent social and political movement, with a reputation as one of Europe's most feared and respected socialist movements. In mid-1928, KPP volunteers, with Soviet training and material aid, began organizing for a coup against the Polish government. This came to pass on the night of September 12th, when Julian Marchlewski, leader of the KPP, declared the establishment of a Polish Soviet Republic at Warsaw. Communist forces marched on the city in large numbers, overwhelming the local garrison. The inadequacy of the government of Ignacy Mościcki was promptly highlighted when the Polish military, ordered to suppress the revolt, mutinied in large numbers. Ejected from the capital and caught in the midst of a furious offensive, the Polish cabinet fled westwards to Germany, seeking with desperation international aid.
Walter von Luttwitz, President of Germany, saw the unrest in Poland as an opportunity to assert Germany's long-standing territorial claims over the Polish (Danzig) Corridor. After an alleged KPP unit attacked the border-town of Gartz, killing eight civilians, the German leadership was able to obtain the support of Britain, France and Belgium for military intervention, "to protect the basic rights of German citizens within the Second Polish Republic"; reluctantly grateful for any sort of assistance against the Communists, Mościcki was complacent. The Polish Corridor was overrun by German troops.
Vicious fighting continued in Poland proper. Most notably, the Communist seizure of Lodz in December 1928 led to a ruthless three-month siege, eventually resulting in 8,000 casualties. The German Luftwaffe (established only two years prior, and consisting mainly of a small number of British-supplied aircraft) made its battlefield debut in Poland, targetting Communist outposts and reeking havoc in Polish cities. This had a devastating effect, contributing to a large number of civilian casualties. Overall, the KPP was unable to replicate the swiftness and aggression of its previous offensives.