Did Russia back out to intentionally let it happen or did they chicken out to avoid fighting the nazis?

It was an attempt of the Polish resistance to avoid being "liberated" by the Soviets to just immediately become occupied by the communist red army. The idea being to liberate Warsaw and get US/UK assistance through the Polish government-in-exile in London to establish Polish military control before the Soviet army arrived. Getting US/UK support could have meant that Poland remained an independent state. Instead, Stalin not only betrayed them, but later actually convicted the surviving leaders of the uprising. "Crimes against communism".

This is now politely referred to as a Soviet betrayal in service of "Stalin's post-war political goals for Poland".

It was definitely not planned with the Soviets, for multiple reasons:

  - the Poles of the AK (London government loyal) were not the communist faction (Lublin government loyal), and saw the insurrection as the last chance to get a Poland out of the Soviet sphere of influence post WWII – especially after the publicization of Katyn;

  - even if they had wanted, Stalin had zero interest in giving a hand to London-loyal Poles that were in frontal opposition to “his” Lublin-loyal Poles;

  - the Germans were not caught flat-footed, they already knew of the insurrection preparations and therefore not only was the city well garrisoned, it would have been in any case, as it was the strategic lock of the area to hold the RKKA on the Vistula;

  - and all the above is moot in any case, because the RKKA units that reached the neighborhood of Warsaw in '44 had as many chance of taking the city as the German units that reached Moscow in '41 – they were just spent and at the end of their logistic tail after months of fighting during the Bagration operation, and had no chance of successfully developing an opposed crossing of the Vistula against two Panzerkorps.

So the London-loyal Poles were in a damned if you do, damned if you don't situation, and at least they were able to go with a glorious bang. Like a Marshal said, “c'est magnifique, mais ce n'est pas la guerre”.

For more context, WWII was started as a partnership between Hitler and Stalin to partition Poland. [0]

Spreading this knowledge is now illegal in Putin's Russia. [1]

[0] https://en.wikipedia.org/wiki/Molotov%E2%80%93Ribbentrop_Pac...

[1] https://en.wikipedia.org/wiki/Law_Against_Rehabilitation_of_... (that name, lol)

We have a monument commemorating children fighting in the uprising: https://pl.wikipedia.org/wiki/Pomnik_Ma%C5%82ego_Powsta%C5%8...

It shows only the better part but doesn't show the bad part. Poles are divided about usefulness of this uprising, how it was (badly) executed and many believe it was deemed to fail.

The aftermath [1] was that ~220k Poles died and out of that 150-200k civilians, often with mass execution - later on a lot of warsaw population was sometimes bitter toward the uprising’s leadership.

To put it in context: within 2 months 200k people died, similar number like in Hiroshima but almost nobody wordwide know about warsaw uprising.

[1] https://en.wikipedia.org/wiki/Warsaw_Uprising#Aftermath