If I enter the battlefield with 5 might and my opponent has 5 units with 3 might, and I use Imperial Decree, do I kill all units or how does it resolve?

No, you do not kill all units — you kill at most 2 of the 5 defending units, and your own unit dies too.

Here's why. During the Combat Damage Step (Rule 460.2.c), you must assign lethal damage to one defender before moving to the next. Your single 5-Might unit assigns damage like this:

• Defender #1 (3 Might) → assign 3 (lethal ✓)
• Defender #2 (3 Might) → assign your remaining 2 (not lethal, but that's all you have left)
• Defenders #3, #4, #5 → 0 damage assigned (you ran out of Might)

Meanwhile, the defenders have 15 total Might to assign back to your single unit — easily assign 5 (lethal). All damage is dealt simultaneously.

Then Imperial Decree triggers: each point of damage dealt to a unit kills it. Your unit (took 5) dies. Defender #1 (took 3) dies. Defender #2 (took 2) dies. But Defenders #3, #4, #5 never took any damage at all — they survive untouched.

Combat result: Opponent still has units remaining, you have none → opponent wins combat, establishes control, and conquers the battlefield (Rules 461.3.a, 461.5).

The key trap: Imperial Decree doesn't let you bypass the lethal damage assignment rule. You still need enough total Might to reach each defender in sequence — Decree only kills them after damage actually lands on them.