The forward youth that would appear
Must now forsake his Muses dear,
Nor in the shadows sing
His numbers languishing.
'Tis time to leave the books in dust,
And oil th' unused armour's rust,
Removing from the wall
The corslet of the hall.
So restless Cromwell could not cease
In the inglorious arts of peace,
But through advent'rous war
Urged his active star:
And, like the three-forked lightning, first
Breaking the clouds where it was nursed,
Did thorough his own side
Marvell presents Cromwell as with an uneasy spirit that seeks the glory of war. “Advent'rous” means risky or daring. 
 The “active star” is an astrological reference to the star that guides Cromwell. In the 17th century, it was widely believed that the alignment of stars and planets had a direct effect on humanity. 
 It was believed that the movement of the cosmos was controlled by divine intelligence or a higher force; this was sometimes, but not always, seen as God. Cromwell is urging his active star which is a complete reversal of the widely accepted beliefs of the time. This also imparts power onto Cromwell as it suggests he has to some extent, the power of God.
An Horatian Ode Upon Cromwells Return from Ireland