When assigning combat damage to multiple defending units, must I assign lethal damage to each unit before assigning damage to another?

Ruling: No. The only mandatory restriction for combat damage assignment is the Tank keyword—you must assign lethal damage to Tank units before assigning to non-Tank units. Beyond that, you can choose how to distribute your remaining damage among defending units and are not required to assign lethal damage to each unit before assigning damage to others.

Sequence:

• Assign lethal damage to all Tank units first
• Distribute remaining damage among other defending units as you choose

Nuances:

• If you have 4 Might attacking into a 5 Might and 3 Might defender (no Tanks), you can assign all 4 damage to the 5 Might unit, leaving the 3 Might unit undamaged.
• After satisfying Tank requirements, you can assign partial damage to one unit even if another unit could be killed with that damage.
• The lethal damage rule prevents overkilling a unit before moving to another, but does not mandate killing every possible target.