The Russian retreat

 Financial Times:

Russians ‘fled like Olympic sprinters’ as Ukraine retook north-east

Offensive was months in the making and relied on western military equipment

 Ukraine used a combined arms operation to overwhelm the "Russians who remained in the area under attack.  The retreat by the Russians was not an organized retrograde operation. but a sprint to the rear.

See, also:

Rapid loss of territory in Ukraine reveals spent Russian military

Morale problems, logistics issues and equipment losses could limit Russian forces to defending the territory they already hold while leaving them open to additional defeats, according to military analysts.