In the Preface to the second edition to Spivak's A Comprehensive Introduction to Differential Geometry, Vol. 2, on p.vii says:
The material in this Volume covers about what I would have completed in my class this term, had political events not intervened.
What political events is he referring to? The preface is signed at Brandeis University, July 6, 1970. The comment is not included in the third edition's preface.
I would prefer a factual account, although speculation is also welcome.
Thank you for your time.