One defuses a bomb.
I think the right answer is: defuse"
As Mr Trilby says, you defuse a bomb and you diffuse tension.
"We have a problem, Houston." (LOL)
According to Merriam Webster defuse is the right word.
caused by his resignation suggests that the tension became widespread as a result of his resignation. diffuse: scatter or spread through.
In Dutch there is an expression:
'to take the fuse from the gunbarrel.' It is commonly used in the figurative sense. Given the interaction between the two languages, it is clearly 'defuse'.
Yes, diffusing is spreading out - you diffuse light to break it into colored bands ...
Diffuse is the should be. it spreads.