What is the origin of "Germany is calling and I must go"?

I saw this phrase on a t-shirt. I found there are a number of different companies selling their version of this shirt. It sounds like a quote from an internet meme or a movie or a video game or something. But I can’t find any source.

Anybody able to tell me its origin? And what the connotation the phrase is seeking to convey?

The original source quote would seem to be “The mountains are calling, and I must go", written by John Muir. I guess it’s a bit of a mashup of that quote with Germany Calling, but I’m not sure what the connotation is beyond that. It just seems to be for fun and perhaps because Germany is a popular backpacking destination?

I just searched for “Switzerland is calling and I must go” and “France is calling and I must go” and found shirts for both.

Also for Florida, Singapore, Chile, etc.

Also a popular theme among those eligible for military service in 1914.

There are all sorts of variants out there for sale, from your favorite sport or hobby to places, like the OP referenced.

This. The asterisk is a wildcard in Google searches, so if you Google:

* is calling and I must go

you’ll get pretty much everything. Check the image results for T-shirts, cards, etc.

Why not London Calling? I live by the river.

That t-shirt would Clash with your pants

That was sort of my point. It’s why I’m wondering about the origin of the phrase.

If it originated as a slogan for a hiking club in 2005, no big deal. If it originated on Waffen-SS recruitment posters in 1940, it has a much different history.