(This is in response to the clarified situation of a R1 UKR (Ukraine)/WR (West Russia) attack with 2 tanks and 2 fighters at UKR being called “held back” as opposed to the UKR/WR attack with 3 tanks and 2 fighters at UKR.)
If you attack UKR with 3 tanks 2 fighters, you have a great chance of capturing UKR. But you will lose about 1 more inf at West Russia, and your tanks at UKR will die on G1, being traded for German infantry. The 2 tanks 2 fighters has a lower chance of capturing UKR, but a bit of good luck results in capture of UKR, and preserving an extra infantry at West Russia, and saving a tank from the German counter. That’s 8 IPC of units. On the other hand, Russia capturing Ukraine with 2-4 units puts pressure on Germany to kill Russia’s valuable tanks, so usually will drain additional German ground units on G1, vulnerable to the R2 counter.
By the way, you sometimes see 3 tanks 1 fighter at Ukraine. The idea is that it’s similar in attack power to 2 tanks 2 fighters, but Russia can afford to “stay” longer (i.e. if there’s 2 Russian fighters and 1 German fighter remaining, the Russians will probably best retreat, but 1 Russian tank and 1 Russian fighter vs 1 German fighter allows Russia to keep going). The second Russia fighter hits WR to increase hitting power there to help preserve an extra infantry.
That’s the SHORT version. For the LONG-WINDED version, see below.
3 tanks 2 fighters at Ukraine can leave Russia with 2-4 ground units surviving, which puts a lot of pressure on Germany. Probably Germany’s best response is to send 2 fighters to the UK cruiser, battleship/transport/infantry/artillery to Anglo-Egypt (fighting UK destroyer on way), fighter/bomber/sub vs UK battleship/Russian sub, fighter to Anglo-Egypt. The fifth fighter is often sent to UK cruiser in dice games as a “safety” (Germany’s gambling with expensive fighters, and losing both fighters would be awful), but with a R1 build of 2 inf 2 art 2 tank (leaving 4 inf 2 art 2 fighter on Cauc, plus surviving West Russia forces plus 2 tanks from Russia), whatever Germany counters UKR with will die on R2 (even if Japan flies what fighters it can in). Since Germany won’t have cheap artillery at Ukraine, that means sending expensive tanks, or committing a fighter. Probably Germany will do best to send a tank and fighter, plus infantry (as opposed to sending a couple of tanks plus infantry, saving Germany a 5 IPC tank). So in sum, Russia hits hard R1 and loses a few extra IPC worth of units on the German counter, but Germany will probably have to take a few chances on G1 to counter to best effect, and will probably have to drain out 1-2 extra infantry or a tank. The Russian recapture of Ukraine on R2 will kill those German units, compensating for the loss of the third Russian tank.
2 tanks 2 fighters at Ukraine risks failure. If successful, Germany can still counter as listed above. But failure to capture Ukraine on R1 can be a real problem, with Russians possibly losing West Russia, Ukraine, and Caucasus, particularly with bad dice at West Russia and/or Ukraine (esp if a fair number of German units survive at Ukraine). The easiest Russia contingency plan is probably Caucasus AA gun to West Russia, and 1 Russian infantry at Caucasus to prevent the 1-German-infantry walk in. The remaining Russian forces stay in Russia. UK can sometimes recapture Caucasus from Persia (with UK air), and Russia can probably counter decently too. (But with a German build of 8 tanks, things get nasty fast, and 4 Jap fighters can land on a German-held Caucasus on J1.)