During World War One, the Netherlands was neutral, but provided a back door for trade for Germany. Why on earth did the Britain put up with this? The Royal Navy guaranteed the safety of the Dutch colonial empire. They could easily take the Dutch holdings in place of say, the Gallipoli campaign. All they had to do was give the nod to the Japanese and they'd take the Dutch East Indies single-handly.(Britain might not want the Japanese to do it for a variety of reasons, but the point still stands) At it's most lenient, the Allies could blackmail the Netherlands to cease trade with the Germans or loose their empire. At the most bold, they could demand the Netherlands declare war on Germany and/or allow passage of troops to flank the German lines.
Why did anything like this never happen during WWI?
P.S. I just realized this sort of logic also applies to Denmark as well. The Allies could take Icelandand Greenland at any time, but the question is if Denmark would laugh at them and let them have a couple sparsely populated ice-covered islands. They were stategicly useful in WWII, but I don't know if they would be in WWI.