When Russia last went to war, in Georgia in 2008, it looked like an easy victory. But Russia's generals were deeply concerned at how badly their forces performed in some key areas of modern warfare.
Russia has spent the seven years since then rearming, re-equipping, and retraining, in order to deal with those deficiencies, and to try to close the capability gap with modern Western armies.
Now the results can be seen in eastern Ukraine, where pro-Russian separatists have gained ground against Ukrainian government troops.
Ukraine's army has not gone through the same intensive modernisation process, and is suffering the effects, facing the newer weapons and systems supplied by Russia.
Two key examples are the use of UAVs (drones) for surveillance and targeting, and the use of electronic warfare.
Both technologies were identified as areas of weakness in the Russian forces in 2008, and both have been intensively developed since. Now, they are in widespread use in eastern Ukraine, placing Ukrainian government forces at a strong disadvantage.
Ukrainian forces are short of secure communications systems. The result is that their communications are both subject to jamming, and often also show their location to Russian direction-finding equipment. This can lead to being swiftly targeted by Russian artillery, including Grad and other, more powerful, rocket systems.
Ukrainian forces are also outclassed by the tanks arriving from Russia. Not only are these more modern than Ukrainian models, but Ukraine is also short of effective anti-armour weapons in working order.
All of these systems, plus medical support and field hospital equipment, are on the list of Ukrainian requests for support, to increase the survivability of their forces when confronting new Russian military equipment.
Losses of Ukrainian aircraft over the conflict zone show how well-equipped the Russian-backed separatists are for air defence. This includes not just the Buk missile system - blamed for downing Malaysia Airlines flight MH17 - but also others like Strela for use at lower altitudes and shorter ranges, and a wide range of lighter, shoulder-launched missiles.
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.