The Neutrals DID have themselves to blame when they were overrun so quickly by the Nazis.
Don't misunderstand my meaning. The neutrals weren't responsible for the Germans deciding to invade. Focus on the last half of this opening statement, the
so quickly overrun part. Once seriously challenged, none of the neutrals displayed any great will to defend their own homelands against invasion, with maybe the possible exception of Norway, and I'll address Norway's case later. Both the Netherlands and Denmark gave up almost immediately. Perhaps a few "honor" shots were exchanged, but in both cases almost without a fight. I'm not faulting any of these countries for not being able to ultimately stop the Germans on their own after a serious attempt at mounting a defense, but they really weren't on their own, and they really didn't attempt to defend themselves in a meaningful way.
The argument of the invincibility of the German military versus Country X, population comparisons, GDP, etc...holds much less weight when you realize that German military was spread all over Europe from Greece and the Balkans to Poland to France and Belgium, and then they decided to invade Denmark, Norway and the Netherlands, finish off France, intimidate the UK into a negotiated peace, and oh, by the way, also invade the Soviet Union. The combined strengths of the UK, France and the neutrals were considerable. The Germans were only able to mount attacks of the neutrals with relatively limited forces, but they correctly judged these would be adequate against a perceived lack of a will to fight, self-imposed isolation, unpreparedness, indecision and disorganization: weakness. They were able to pick them off cheaply one-by-one. If the neutrals had decided to fight with the weapons and forces they had on hand (including reservists) until they just couldn't fight anymore, and if they joined in with the Brits and French once forced into the war, the outcome probably would have been different and not have ended so favorably for Germany.
The Belgians began to fight along with, but not in close coordination with, the BEF and France, but they capitulated in less than 3 weeks without any warning to the UK or France. This left the BEF and French forces in a very bad position that greatly contributed to the Fall of France and the near destruction and capture of the BEF. While the French weren't neutrals, they should be faulted for their abysmal performance and premature capitulation (mostly due to very poor top-level military and civil leadership). Holland gave up after only four days when several key forts were captured by lightly armed glider troops and the terror bombing of Rotterdam achieved the goal of victory through intimidation without any major ground actions having to take place. When German motorized forces ran border checkpoints shortly before dawn, the Danes just decided they'd had enough and called it quits before lunch. This was key to the success of the Norwegian Invasion, for without being able to use Danish airfields to quickly reinforce Norwegian bridgeheads, the German plan to capture Norway would have failed.
And then there was Norway, whose successive Governments had severely neglected its military for decades. Materiel was lacking and outdated, training had lapsed, the High Command was formed of an elderly "retired on active duty" officer cadre incapable of grasping modern warfare concepts or the ability to make rapid decisions, and they had an avowed pacifist Minister of Defense. Still, of all the neutrals, Norway had the best chance of stopping the Nazi's Northern European advance. Norway had sizeable reserve forces who could use the restrictive terrain and waterways to favor the defense. There were well placed coastal forts covering all major harbors whose guns, while mostly old, could still adequately provide close range defense when coupled with the pre-planned minefields (and a few secret underwater torpedo batteries).
The keys to stopping the German invasion would have been to prevent the Germans from establishing the initial sea and air port bridgeheads, prevent German reinforcements from landing and organizing and finally, to call on and fight with British and French assistance to expel the invaders. This was within Norwegian capability with the forces and materiel they had available in April 1940, but it didn't happen due to non-existent civilian and military national leadership lacking a steadfast will to fight. The key moment in the recent Norwegian film, "The King's No!, The King's Choice in English" was the scene where the lily-livered Cabinet was pressuring King Haakon to surrender and accept the German terms, like his brother the King of Denmark had done several hours before. The King's refusal was the beginning of top-level Norwegian resolve to resist the invasion, but it came too late to save the country.
Warnings of an impending German invasion from several credible sources were ignored. Multiple pleas for a mobilization were rebuffed by the Cabinet and National Command Staff in the weeks before the invasion. Sea mines remained unplanted in depot storage, but there was an order to begin laying the Oslo fjord minefields the week after the Germans invaded. There was only limited or no heavy caliber coastal artillery ready ammunition stored near the guns. Coastal batteries were mostly only manned by caretaker or skeleton crews insufficient for operation. Army reserve units were mandated to store their small arms, bolts and ammunition in separate depot locations. Early in the evening on the night before the invasion, the Command Staff and Cabinet began receiving reports of unidentified darkened ships closing on several key harbors, including a multi-ship convoy steaming up the fjord leading to Oslo. There were later reports of gunfire, possible sinking of Norwegian guard vessels, and even that German was being spoken on the unidentified vessels. Still, the Cabinet and High Command failed to issue orders to coastal defenders, but finally did agree to initiate a reserves mobilization by mail that could take two weeks, as to not unduly alarm anyone.
Some local commanders engaged the invaders on their own initiative and achieved initial success, but for the most part, the invaders landed unopposed, established their bridgeheads and began receiving reinforcements flown in from Denmark. My wife's father recalled Army reservists not waiting for the recall notice to arrive in the mail. They just showed up at their mobilization centers when they heard on the radio that the Germans had landed, but they were sent back home by the confused reserve cadre with instructions to come back in a day or two after the Army Staff had time to sort things out.
By the time the Army Staff had sorted things out it was too late. The Germans were firmly established, well organized and were gaining strength every day to the point they couldn't be stopped.