How does the word order work in the second sentence?

Question

How does the word order work in the second sentence? Why is the word "mal" in the second position instead of a verb?

Niemand verlässt die Formation. Nicht mal dann, wenn einer von uns getötet wird.

Answer 1

It's a truncated sentence.

The full version would be:

• "Nicht mal dann [verlässt jemand die Formation], wenn einer von uns...."

"nicht mal dann" is one element, a time element to be precise. It's pretty empty content-wise so it gets specified in the "wenn"-sentence later on.

But it constitutes position 1 as a whole, and then would come the verb, but in this case, the rest of the main sentence is simply missing.

Answer 2

"Nicht mal dann" would be taken as one word and as the above comment mentions, the hidden phrase is [verlässt jemand die Formation]. So "verlässt", a verb, is in the second position and hence the word order fits.