So as I mentioned in the last post, these cups were given for things that someone has done - it's a celebration of finishing something. So, what kind of stuff are they celebrating?
Well, turns out the reasons are as varied as there are cups out there. Granted, the most common reason is obvious - someone finishing their military service. These have a few variations - discharge commemoration (除隊紀念), leaving the service commemoration (退營紀念), or victorious return commemoration (凱旋紀念). The vast majority of military sake cups are related to these three, which celebrate someone finishing their term of service or just came back from a military campaign. The cups could get more specific - unit number, particular events, years, etc, but the basic reason is the same.
There's another question though - who gives the cups? Well, this is more interesting. From what I have gathered, there are basically three types of people who would give you cups if you were a returning soldier. They are
1) Friends and family
2) Colleagues in the military
3) Unit-wide commemorative items
Friends and family is easy to imagine - you are a returning soldier, and you just got home. Your family and neighbours are very happy that you got back alive and safe. Everyone is in the mood to celebrate, and some of your friends or your family go out and buy some of these commemorative cups from shops that specialize in them to help celebrate this event. They might splurge and buy something nice, or just something really basic, but either way, it's a nice gift for someone who just came back.
The second is that if you're leaving a post, especially if you're an officer, oftentimes your colleagues who have worked with you for a few years or more are more than happy to give you something to remember them by. This is more common for nicer, officer level cups - fellow officers who probably shared the same mess were happy to pool together some money to give you a going-away gift. Maybe you got promoted, or transferred to a new unit, and the cup is meant to preserve and remember precious friendship. These cups are often quite nice and recorded with words that tell you it's from a group of officers. A particularly nice example is this lacquer cup with matching stand.
The third reason is that this is some sort of unit-wide issuance of commemorative items. This is more likely for victory cups of all kinds or cups related to unit events, like those regimental banner days when a unit celebrate the day they received their regimental banner, or participation in military exercises, or other similar kinds of one-off events that are united related. It would be weird, for example, for one soldier to go out and buy a cup related to having practiced water-fording with horses. It makes more sense for a unit to collectively buy cups related to this and then everyone who participated would get a set of five or something like that.
We have proof of this, because one of the cups I'm selling right now tells you exactly why this sort of thing happened. This is a mountain artillery unit that participated in the Shanghai Incident fighting in 1932. The lid of the box tells us that the unit received reward money from the emperor himself. Instead of having the soldiers all just get their cash and use it for whatever, the unit commander decided it would be nicer if they pooled the money and got these pewter cups made to commemorate the victory. Not sure how the ordinary soldiers appreciated this, since they probably wouldn't have minded extra cash, but we now have some really nice pewter cups as a historical relic of this event and the exact reason why they were created in the first place.
There are other reasons too that I've seen, but those are less common. For example this cup that celebrates the opening of a new training hall for officers. This is probably just a cup made for those involved in the construction of this new hall for officers of this particular unit.
There are other reasons too, far too numerous to name really for an exhaustive account. Suffice to say, I keep discovering new things - the water-fording cup, for example, was a true revelation.