Defensive is when the enemy come's to fight the muslim's, but offensive is when it's for the purpose of da'wah.
for example:
defensive: badr, uhud, khandaq. [when the enemy comes to fight the muslims]
offensive: in ash-Shaam, Persia etc. [to spread islaam in further area's of the world]
wa Allaahu a'lam.
__________________