I’m not sure if there was anything conflicting between what Krieg and I said.
You must be at war to move into a friendly territory. The Dutch territories are friendly to the Allies. Therefore, an Allied power must be at war to land a plane on a Dutch territory.
The UK and Anzac both start the game at war (with Germany, even on the Pac board alone) and therefore may immediately land planes in Dutch territories, even if not yet at war with Japan. The US can land a plane or unit on the DEI as soon as they’re at war with any axis power. Russia may land a plane or unit there as soon as they are at war with Japan, specifically.
It’s important to note that only Russia has split theater neutrality conditions.