Not 100% sure of the wording of this, maybe it should be "if the Sirians try to invade," "when" sounds like it's inevitable.
I might try some variations on WW1 recruiting posters too.
Monday 12.30 PM - I've changed it to:
Where will YOU be
When the Sirians attack?
The Imperial Navy and Marines need YOU!
Join the fight TODAY!
I think the original was too verbose, this feels a bit more like a real recruiting poster. Thanks to autopope for the header, and everyone else for their comments.