This was a rule that was added because of the Axis advantage in earlier games being able to bridge. From Italy to Africa and Japan to Korea, the axis had great advantage with that plus a defense of 1 so I assume transports at 0 forces the axis powers to actually commit a naval defense.
air unit rules
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Can I do an amphibious assault on an island with only aircraft?
And can those aircraft also attack the sea zone surrounding the island? -
- This question is rather ambiguous. If you’re attacking with aircraft only (no land units), it’s not an amphibious assault. If you mean, “Can I conduct an amphibious assault without cruisers or battleships for shore bombardment?”, the answer is yes, you can.
- When conducting an amphibious assault, you have to choose which aircraft will participate in any naval battle and which will participate in the land combat; they cannot do both. Any naval battle must be resolved before amphibious landings can take place.





