World War II began on September 1, 1939, when German troops attacked the Polish military depot on the Westerplatte peninsula in the port of the Free City of Gdansk. The small garrison fought bravely, inflicting three times as many casualties on the Germans as it had suffered itself. for seven days, but had to surrender after six days.

The Battle of Kutno (or the Battle of the Bzura), from September 9-13, was the bloodiest battle of the German invasion of Poland. The Army Poznań, led by Generał Tadeusz Kutrzeba, launched a counter-offensive against German forces advancing on Warszawa. Initially successful because of the Poles’ greater numbers and superior artillery, the Army Poznań was unable to defend itself against German Panzer tanks and aircraft. While German losses included 8,000 dead, 17,000 Poles were killed and 180,000 Polish troops were captured.

The Siege of Warsaw lasted from September 1 until September 27. During the first week, German aircraft ruthlessly bombarded the city. By September 8, German forces had reached the city’s suburbs, but were initially repulsed by the Polish Army. After surrounding the city, German subjected the city’s residents to endless artillery bombardment starting September 22. By the time Generał Tadeusz Kutrzeba surrendered the city on September 28, many of its buildings had been destroyed and 40,000 civilians killed. In addition, 140,000 Polish soldiers were captured by the Germans.

The Battle of Kock, which took place on October 2-5, was the final battle of the invasion of Poland. The Special Operational Group Polesie, a Polish Army Corps under the command of General Franciszek Kleeberg, fought the German XIV Motorized Corps, led by General Gustav Anton von Wietersheim. Running low on ammunition, General Kleeberg surrendered to the Germans on October 6. This battle was less bloody than other battles, with only several hundred casualties on both sides. However, while 17,000 Polish troops were captured, a small number escaped to form the first partisans of the war.

In early 1940, the Allies prepared an expeditionary force to secure the Norwegian port of Narvik in the hope of sending aid to Finland, which was invaded by the Soviet Union in November 1939, and preventing the Germans using it to import Swedish ore for its war industries. However, Finland surrendered to the Soviets on March 6, 1940, and the Germans launched an invasion of Norway on April 9, during which they captured Narvik’s garrison. Thus, the Allied forces were dispatched to recapture Narvik. These forces included the Polish Navy and the Polish Podole Brigade transferred from France, because the Polish government-in-exile was determined to prove that Poland was still strong enough to continue fighting and worthy of a seat on the Allies’ Supreme War Council.  Prior to the German invasion, Norway’s coast was patrolled by Polish and British naval ships. On April 8, the Polish submarine Orzel sank the German ship Rio de Janeiro, which had been transporting several hundred German troops who were part of the invasion force. On April 10 and 13, the Polish Navy and the British Royal Navy together sank 10 German destroyers participating in the landing of troops at Narvik. Between May 9 and 28, the Polish Podole Brigade was part of an international force, including Norwegians, British, French, that  recaptured Narvik from the Germans. But by then, Allied leaders had decided that these troops were needed in France to stem the German invasion, and Narvik was abandoned.

Following the fall of France in June 1940, the last standing part of the Polish army was the 1st Polish Independent Carpathian Rifle Brigade. It had been formed in French-controlled Syria, but after the local French military commander announced his support for Vichy France, the French rump state that collaborated with the Nazis, the Carpathian Rifle Brigade crossed the border into British-controlled Palestine in order to continue fighting the Germans. Following the landing of a German force in Tripoli, Libya, in February 1941, the British sent the Polish brigade to Mersa- Matruh to protect the path to the Nile Delta. By the summer of 1941, however, the Germans had advanced further and bypassed the port of Tobruk, leaving its British garrison facing a siege that would last 241 days. In August, the Polish brigade secretly relieved the Australian regiment. In late November-early December, as part of Operation Crusader,the Poles played a part in a daring breakout from the garrison that joined up with the British Eighth Army and lifted the siege. Eventually, the British brought the Carpathian Rifle Brigade back to Egypt to be part of a larger Polish force in the Middle East that was to be used in a future major Allied campaign.

Situated 90 miles south of Rome, the Benedictine monastery of Monte Cassino sat atop a massif in the center of the Gustav Line, a series of German defensive structures designed to block the Allies’ advancement to Italy’s capital city and further north. Its capture by the Allies required four different battles between January 17 and May 18 of 1944. Eager to reinforce its international credibility, the Polish government in exile approved the use of the Polish II Corps during the fourth battle, which began on May 11. The fighting around Monte Cassino for the Poles was bloody as German artillery had a clear view of any Allied offensives. In addition, the men of II Corps were short of rations when they arrived at the front and were forced to hole up in foxholes from previous assaults. The casualties as they ascended the mountain facing a barrage of German fire during the next six days.  Eventually, a thirteen man patrol from II Corps reached the monastery on the morning of May 18 and seeing it empty, hoisted an improvised Polish flag atop the ruins. The Germans had withdrawn after a battle that caused the deaths of 860 Polish soldiers. 

The Allies’ victory in the Battle of the Falaise Pocket on August 12-21, 1944, two months after the D-Day landings on Normandy beaches, opened the path to Paris and that city’s liberation from the Nazis. A German counter offensive launched on August 7 had failed and the German Army B was at risk of being surrounded and captured in a pocket near the town of Falaise by advancing American, British and Canadian forces. The Polish 1st Armored Division, fighting with the 1st Canadian Army, was tasked with sealing a 12 mile-wide gap in the pocket through which German forces sought to escape. On August 20-21 the German Army B attacked the Polish troops on Hill 262 overlooking the escape route. Fighting desperately, the Poles nearly ran out of ammunition before Canadian forces arrived to seal the gap. Polish casualties included 325 killed and over 1,000 wounded, but for the Germans, 10,000-15,000 were killed and half of the remaining 100,000 soldiers were captured after the pocket had been sealed.