Can I fail to meet the minimum necessary effective military strength?
Solution 1:
The worst possible ending is only stated to be < 1,750 points, which, depending on if you play multiplayer or not, could be hard, or easy to attain.
If you don't play the multiplayer at all, your Galactic Readiness Rating (GRR) will stay at 50%, meaning your War Assets rating will be multiplied by 0.50 to calculate your Effective Military Strategy.
If you have a GRR at, or below 3500, it will be halved and thus be below 1750. It's never going to be possible to be at 0, simply because the main quests give you War Assets.