Suit. If somebody wanted either black tie or white tie, they would let you know about it very clearly. I could imagine that "semi-formal" is even blazer and tie territory - but a suit is clearly the safest route.
My guess (and I'm sure somebody could clear this up) is that "semi-formal" has a very specific dress code in military circles (i.e. there may well be a 'semi formal' uniform), and that's the target audience.