This is from the bit in Book 2 where the various devils are putting their cases to Satan about what they think should happen next.
Belial is speaking - and arguing that he would be very much in favour of "open war", except that the reasons for going for war displeased him. Heaven is well armed, he argues - and there will be no way
...what Revenge? the Towrs of Heav'n are fill'd
With Armed watch, that render all access
Impregnable; oft on the bordering Deep
Encamp thir Legions, or with obscure wing
Scout farr and wide into the Realm of night,
God has heaven well guarded, Belial argues. There is another option, he says, which is that they try and break their way in by force:
Or could we break our way
By force, and at our heels all Hell should rise
With blackest Insurrection, to confound
Heav'ns purest Light...
However, God would still win: he'd expel them again, and purge off their fire, proving ultimately victorious:
... yet our great Enemy
All incorruptible would on his Throne
Sit unpolluted, and th' Ethereal mould
Incapable of stain would soon expel
Her mischief, and purge off the baser fire
Beaten in this way, ("thus repuls'd") their only, and final hope, is dull, flat, straightforward despair:
Thus repuls'd, our final hope
Is flat despair...