#1 It allows Russia to move into all Allied territories on the Pacific map (except USA, until USA is at war)
#2 It of course allows Russia to attack Japanese territories and/or units
#3 It allows Russia to attack neutrals on the Pacific board only (Mongolia, which would normally be stupid, or Afghanistan)
#4 It allows Allies to move units into Russian territories (except USA until USA is at war). It’s not a common strategy (or even optimal), but the US can theoretically start a bombing campaign from the “relative” safety of Amur, tempting Japan to attack to activate the mongolians.
To me, this is the best reason to immediately DOW R1, because there’s no punishment until Russia actually performs a combat move against Manchuria, so you mays as wells let the US have the option to start using Russia as a staging area.
And also, to be perfectly clear:
A DOW is required to perform any of those 4 items and is intended to be declared PRIOR to the combat move phase. Most gaming groups or partners will allow a DOW to happen simultaneously. Moving a unit from Russia into China (for example) is NOT considered a Declaration of War but it does require a DOW prior to combat move phase. They are not one and the same. If you didn’t declare war prior to combat moves, you cannot move a unit into china during noncombat. Again, most gaming groups will forgive the oversight, but technically…
Further, a DOW does NOT require a combat move be performed, nor does it require a non combat move into an allied space. For example (and this is very hypothetical) if Russia is at war with germany, and wants to do some airstrike against german units in Persia or somesuch AND then wants the option to land planes in india if the fight goes south. To do so they MUST declare war on Japan PRIOR to the combat move so that the planes can land in India as a noncombat move. They need that DOW first, even if they want the option to do that and aren’t even intending to land in india if the fight goes well or some other battles go poorly.
there… done editing.