At present, we have only a few tags concerning military matters that I can find:  

[tag:military-records]x111  
"For questions about documents pertaining to military personnel or units, including service records, muster rolls, medal rolls, casualty lists, regimental log books, etc."  
[tag:military] is a synonym of this tag.

[tag:navy]x20  
"The sea-going arm of the military forces of a country."

[tag:british-army]x14  
"Used with questions concerning the records created and kept concerning the military forces of the United Kingdom."

[tag:us-marine-corps]x2  
"A branch of the United States Armed Forces. This tag should be used for questions about finding or obtaining records of this branch, the meaning of parts of the record, or other questions pertaining to genealogy and the Marine Corps."
	
The use of [tag:army] and [tag:air-force] as synonyms of [tag:military-records] was discussed above, but these appear no longer to exist, while [tag:navy] is not a synonym but stands alone.

So right now, it's a bit of a mess. I have used the "british-army" tag as it seemed appropriate for my questions, without ever considering if it is actually an appropriate tag to have on the site. "british-army" _does_ seem like a "useful" tag, where I can click on it and see all the closely-related questions that may well be of interest. And that's surely the point of a tag. But it doesn't seem right to have "british-army" without having "us-army" and all the others, and that would tag overload. "us-marine-corps" has much the same problem. 

"military-records" is fine and useful, but doesn't really cover questions that don't directly and solely concern records, such as [Why would a regular soldier join a militia?][1], which asks if such a transfer was likely. Given that @JanMurphy and @HarryVervet have helpfully been cleaning up tags for [baptisms and marriages][2] and [burials and cemeteries][3], using "topic-records" and "topic-practices", I suggest we aim for consistency with that effort by adding [tag:military-practices] to cover those subjects? 

Reinstating army, navy, airforce (and marine corps?) as full tags without national affiliations would allow simple filtering to related records (as @JanMurphy [noted][5] for the navy tag), . The "related tags" field on the right of the screen then [allows cumulative filtering][6] (e.g. "military-practices" then "army" then "british"), and should achieve much of the utility of the "british-army" etc tags.

[1]:http://genealogy.stackexchange.com/q/12462/4485
[2]:http://meta.genealogy.stackexchange.com/q/3198/4485
[3]:http://meta.genealogy.stackexchange.com/q/2098/4485
[4]:http://meta.genealogy.stackexchange.com/q/1966/4485
[5]:http://meta.genealogy.stackexchange.com/a/1965/4485
[6]:http://meta.stackexchange.com/a/17008