I thought the character "を" (wo) was only used for the particle whose only job was to indicate the direct object of a verb.
But today I saw it at the end of an exclamation on a sign I think on a shop:
So what job is を doing here?
It's still the object marker. The sentence is just not finished and the verb is implied.
(there was a question mentioning suspended sentences but I cannot find it for the moment)
Anyway it's often used:
May the force be with you!
Additionally it gives some kind of propaganda feeling to the sentence.
It's just an ellipsis of the verb. It happens too with other particles, for example, you have "復興へ！" (towards reconstruction!) here and there in the Tohoku area.
I think that it is mostly used in an incentive context, to express "let's all…"