My assumptions: Diocletian or Maximian Alexandria mint "MILITUM" is the missing word on the right of the reverse. The officinal mark is a gamma maybe or something resembling that.
@Valentinian had a thread late last year which showed how the tetrarchal portraits are more distinguishable than traditionally thought so maybe the portrait profile might be of some use. Maximian seems to always have at least a slightly upturned nose, or sometime a bulbous tip on his nose, and somewhat shorter (bridge to tip) than the other tetrarchs. Your coin's profile looks like Diocletian. I'm not an expert on these types but the portrait style doesn't look like Alexandria yet that's the only possibility I can see from the mint mark. Hmm. Perhaps there are other more concreted ways of making the determination for your broken coin (did both issue this type at that mint, for instance), but my guess is Diocletian .
Well, the last letters of the legend are ANVS AVG so technically it could be Diocletian or Maximian. It could not be Galerius or Constantius. I would favor Diocletian as @TIF points out. Anyway, nice example of an argenteus (even with the broken piece)!