Agree with Ike here: it really depends on what the rest of the board looks like.
Some options from my own experience:
1. If Moscow falls G9 this means Germany has channeled a lot of resources east.
Strong counter for the allies is to land 40+ units in western Europe. Most likely even Western Germany and then push back west (liberating France) and south (taking out Italy). In the meantime Berlin should be maxed out every turn. Of course Japan must also be prevented a win in the Pacific first priority (defensive spending).
If the allies went ‘Japan First’, this option may not be available to you since it requires minimal spending in the Pacific (i.e. a defensive budget rather than an offensive one).
2. If Moscow falls G9 after the allies went ‘Japan First’ (assuming CaÃ¯ro is still allied controlled), the allies must of course start channeling more untis into CaÃ¯ro/Middle East than the axis can. And/or into London, depending on what the axis targets are. Hopefully they are able to do so and start pushing the axis back. Alternatively you can try to liberate another VC. If the allies can build up a huge invasion force/shuck from Gibraltar, they have the flexibility to try to liberate Paris or aid the defense of CaÃ¯ro.
Defeating Germany after Moscow fell with no meaningful allied threat in the west already in place is no small thing to do…
As the Axis I have noticed that hunting down VC’s too aggressively really means putting their heads on the block (like in option 1 above). IMHO for the axis it is best to be patient. Aim to stabilize economic balance and then jump in the gap the allies have left you. A few examples:
3. If the allies go Europe First (while still preventing Japan a 6VC win), Germany best prevent the allied landings at all costs. This takes away a LOT of resources from the eastern front so Moscow should be safe (except when Russia built too much ‘wrong’ units). In Russia, Germany + Italy need to aim for a defensive perimeter as far eastwards as possible in order to be able to contest or defend rich Russian territories.
Japan should secure India if it doesn’t already have it under control and then make a deep strategic calculation (4/5 turns ahead); will it be possible to grab Hawaii/Sydney if the USA defends it with all its resources while Japan goes all-in to take it and hold it? Will Japan also be able to hold all other VC’s? Could Japan ‘trick’ the USA and then suddenly swing east when it is too late or is the USA-player a bit Pacific-ignorant and do you think he/she will continue that?
If Yes, go grab this last VC. Otherwise, just match the allied (USA) forces and offensive spendings in the Pacific and send the rest to contest the Middle East/Africa. From India (Naval Base), Japan can invade Africa in 1 turn. If Japan cannot take it, can Japan convoy it? I once lost too many indian ships too soon and as a result, got convoyed all the way from the Persian Gulf to the Brazilian coast by Japanese submarines…
I could go on and on with options, particularly about allied strategies for late game ‘JF’ course of events and the axis responses but I think other players have more experience with this and I already wrote quite enough for now ;-).