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Summer 1944

Wehrmacht planners were convinced that the Red Army would attack again in the south, where the front was 80 kilometres (50 mi) from Lviv and offered the most direct route to Berlin. Accordingly, they stripped troops from Army Group Centre, whose front still protruded deep into the Soviet Union. The Germans had transferred some units to France to counter the invasion of Normandy two weeks before. The Belorussian Offensive (codenamed Operation Bagration), which was agreed upon by Allies at the Tehran Conference in December 1943 and launched on 22 June 1944, was a massive Soviet attack, consisting of four Soviet army groups totalling over 120 divisions that smashed into a thinly held German line.

They focused their massive attacks on Army Group Centre, not Army Group North Ukraine as the Germans had originally expected. More than 2.3 million Soviet troops went into action against German Army Group Centre, which had a strength of fewer than 800,000 men. At the points of attack, the numerical and quality advantages of the Soviet forces were overwhelming. The Red Army achieved a ratio of ten to one in tanks and seven to one in aircraft over their enemy. The Germans crumbled. The capital of Belarus, Minsk, was taken on 3 July, trapping some 100,000 Germans. Ten days later the Red Army reached the prewar Polish border. Bagration was, by any measure, one of the largest single operations of the war.

By the end of August 1944, it had cost the Germans ~400,000 dead, wounded, missing and sick, from whom 160,000 were captured, as well as 2,000 tanks and 57,000 other vehicles. In the operation, the Red Army lost ~180,000 dead and missing (765,815 in total, including wounded and sick plus 5,073 Poles), as well as 2,957 tanks and assault guns. The offensive at Estonia claimed another 480,000 Soviet soldiers, 100,000 of them classed as dead.

The neighbouring Lvov–Sandomierz operation was launched on 17 July 1944, with the Red Army routing the German forces in Western Ukraine and retaking Lviv. The Soviet advance in the south continued into Romania and, following a coup against the Axis-allied government of Romania on 23 August, the Red Army occupied Bucharest on 31 August. Romania and the Soviet Union signed an armistice on 12 September.

The rapid progress of Operation Bagration threatened to cut off and isolate the German units of Army Group North bitterly resisting the Soviet advance towards Tallinn. Despite a ferocious attack at the Sinimäed Hills, Estonia, the Soviet Leningrad Front failed to break through the defence of the smaller, well-fortified army detachment "Narwa" in terrain not suitable for large-scale operations.

On the Karelian Isthmus, the Red Army launched a Vyborg–Petrozavodsk Offensive against the Finnish lines on 9 June 1944 (coordinated with the Western Allied Invasion of Normandy). Three armies were pitted there against the Finns, among them several experienced guards rifle formations. The attack breached the Finnish front line of defence in Valkeasaari on 10 June and the Finnish forces retreated to their secondary defence line, the VT-line. The Soviet attack was supported by a heavy artillery barrage, air bombardments and armoured forces. The VT-line was breached on 14 June and after a failed counterattack in Kuuterselkä by the Finnish armoured division, the Finnish defence had to be pulled back to the VKT-line. After heavy fighting in the battles of Tali-Ihantala and Ilomantsi, Finnish troops finally managed to halt the Soviet attack.

In Poland, as the Red Army approached, the Polish Home Army (AK) launched Operation Tempest. During the Warsaw Uprising, the Red Army were ordered to halt at the Vistula River. Whether Stalin was unable or unwilling to come to the aid of the Polish resistance is disputed.

In Slovakia, the Slovak National Uprising started as an armed struggle between German Wehrmacht forces and rebel Slovak troops between August and October 1944. It was centered at Banská Bystrica.