The British, however, were showing signs that they were not going to always respect a neutral waterway when it would benefit the Germans. This can be seen with the Altmark as well as the seizure of German coal being shipped to Italy which forced the Germans to continue those shipments only through land routes.
With the prospect of the neutral waters of Norway no longer meaning anything coupled with the talk of supporting Finland by sending units through Norway and Sweden it is easy to see how Hitler may at the very least have speculated that an Allied move on Norway was at the very least in the works (and he would have been correct). In this case, the British were a day late and a dollar short, but it will provide the location of the first armed clash between British and German soldiers. This will take place at Lillehammer.
But German Admiral Raeder's first recommendation to Hitler about invading Norway came in the fall of 1939 -- iirc, even before Stalin invaded Finland on November 30.
This lead to the first meetings of Hitler, Raeder and Norwegian Vidkun Quisling starting December 11.
"During a second meeting with Quisling on 18 December, Hitler reiterated his desire to keep Norway neutral but indicated that should Allied forces extend the war to Scandinavia, he would counter appropriately. "
And just who would decide if the allies had extended the war?
Why Hitler, of course.
For months Sweden and Norway declined the allies' offers of military aid for them and Finland, so Hitler had no pretext. But he didn't need much -- didn't need an actual allied plan, or preparations for invasion. All he needed was a pretext, and that came on February 16, with the Altmark Incident.
On that date the British destroyer Cossack entered a Norwegian fjord and freed about 300 British POW's held captive on the German tanker Altmark.
"Hitler ordered that the development of invasion plans be sped up. He did so to obtain assurance against Churchill's already existing plans to draw the Norwegians into the war and take control over the important harbour of Narvik."By 21 February, General Nikolaus von Falkenhorst was placed in charge of planning the invasion and command of the land-based forces."
Churchill's plans?
All militaries have "plans" for every contingency. So the issue is not, "did Churchill have plans?"
Rather the question is: was there any evidence outside Hitler's fertile imagination that the Chamberlain government intended to invade Norway and Sweden against those countries' objections?
Answer: None that I know of.
And yet Hitler ordered Operation Weserübung by February 21.
"Already in low-priority planning for considerable time, Operation Weserübung[6] found a new sense of urgency after the Altmark Incident."
The Soviet Finnish Winter War ended on March 13, 1940.
"With the end of the Winter War, the Allies determined that any occupation of Norway or Sweden would likely do more harm than good, possibly driving the neutral countries into alliance with Germany. "
So, as of March 13, the Germans have been working on Norway invasion plans at low level for two months and at a high level of one month.
By contrast the Chamberlain government is still just twiddling its thumbs.
Then at last, the agitations from France and from Churchill moved the Chamberlain government to hastily draw up plans to land forces in Norway, starting April 8.
But the Germans with three months head start in preparations, began their invasion on April 3, landing forces in Norway by April 8.
My point here is: the Germans were always well ahead of the bumbling British Chamberlain government, and that's why their invasion was successful.
So the claim that Hitler was driven to invade Norway by British preparations is not supported by evidence I've seen.
Finally, consider the vital importance of Norway, not only for Germany's war materials, but also as a base for submarines and bombers to attack allied supply ships in the Atlantic. This fact was not lost on Admirals Doenitz and Raeder.
Bottom line: Hitler did not need Britain to supply him a "reason" for invading Norway. He already had all the reasons in the world. What he only needed was any old pretext, which the Altmark Incident on February 16 gave him.
A blood thirsty murderer does not need a "reason" to kill, only a likely victim, which Norway was, imho.