Embeddings of maximal tori in classical groups and explicit Brauer-Manin obstruction

Embeddings of maximal tori into classical groups over global fields of characteristic not 2 are the subject matter of several recent papers, with special attention to the Hasse principle. The present paper gives necessary and sufficient conditions for this embedding problem, and in particular for the Hasse principle to hold. Using work of Borovoi, this is interpreted as a Brauer-Manin type obstruction.