Firstly, Horatio refers to the ghost of Old Hamlet as being in "fair and warlike" form, and, moreover, uses "march" as a verb, perhaps implying some sort of military marching:
What art thou that usurp'st this time of night,
Together with that fair and warlike form
In which the majesty of buried Denmark
Did sometimes march?
Horatio later recognises Old Hamlet in the ghost by his armour, and remembers his military deeds:
Such was the very armour he had on
When he the ambitious Norway combated.
So frown'd he once when, in an angry parle,
He smote the sledded Polacks on the ice.
Then, after the ghost has gone, with his "martial stalk", Marcellus asks his companions
...tell me, he that knows,
Why this same strict and most observant watch
So nightly toils the subject of the land,
And why such daily cast of brazen cannon,
And foreign mart for implements of war.
Marcellus has noticed that the military preparations are being stepped up, and, as a watchman, notices that there are far more watchmen set on. He knows that Denmark is preparing for a war.