Possible Clues:

• Sudden incursion
• Advance made at the expense of another
• Means of encroachment
• Radio broadcast about end of Martian invasion
• An invasion or hostile attack
• Attack route towards city centre, say?
• Bit of headway
• Attack — no raid (anag)
• Encroachment — ordain (anag)
• Raid or no raid -- that needs sorting out