Nazi Germany invaded Russia in the Operation Barbarossa on June 22, 1941 — for the Soviet Union marking the begin of the Great Patriotic War. Due to enormous miscalculation by Joseph Stalin and the other leaders of the Soviet Union, the killing of most of the Red Army's top leadership in the aftermath of the Great Purge, and the chaotic state of Soviet defence readiness, German progress was rapid and by September the Wehrmacht had invested Leningrad.
Unable or unwilling to press home their advantage and with a hasty but brilliant defence of the city organised by Marshall Zhukov, the Germans laid siege to the city for 900 days. The carnage in the city from shelling and starvation (especially in the first winter) was appalling but Adolf Hitler was never able to hold his proposed victory party in the city nor carry out his planned destruction of this jewel of European civilisation.
In the chaos of the first winter of the war no evacuation plan was available or executed and the city quite literally starved in complete isolation until the deep winter when an ice road over Lake Ladoga was established.
The bravery of the city's defenders was an important symbol of the Soviet will to resist - in the first few weeks of the war the British had been so disheartened by the collapse of the Soviet armies they thought a Nazi victory was all but inevitable. Most famously Dimitri Shostakovich's Seventh or Leningrad Symphony was written and first performed in the besieged city in 1941. The symphony became immensly popular in the United States and, as weapon of propaganda, was a highly effective symbol of the by then global struggle against fascism.
The warnings to citizens of the city as to which side of the road to walk to avoid the German shelling can still be seen (having been restored after the war).
As a sad postscript, Stalin had the leaders of the city executed on various pretexts after the war - they had, through their bravery and courageous defence, earned a respect of the citizens that the dictator resented and feared.