Independent experts, as well as Nato, Western leaders and the Kiev government, say there is clear evidence of direct Russian military involvement, despite Russian denials.
As part of its military transformation process, Russia has been practising for conflict with an intensive programme of exercises and manoeuvres, involving tens of thousands of servicemen across the country.
These exercises have been increasing in size and complexity, and often have a storyline which is directly hostile to the West.
Now, in addition, Russia has the benefit of a live testing ground in eastern Ukraine, where it can try out its new weapons, systems and tactics. The results - especially if all of these are tested against any potential new US defensive systems supplied to Ukraine - will help Russia assess how its forces would fare in a direct confrontation with Nato.