JEFFERSONVILLE, N.Y. — On the day his law license was reinstated this past summer, Stanley L. Cohen got a call from an old friend and client, Mousa Abu Marzook, a senior political leader of Hamas, the militant Islamic group that controls Gaza.

“He said, ‘You’re up to trouble again already?’” recalled Mr. Cohen, 67.

In certain circles in the Middle East, he said, “Word had gotten around very quickly that I was back.”

That Mr. Cohen is back — after a prison sentence on federal tax charges that resulted in the suspension of his law license — is certain to infuriate many people.

He has spent much of his 35-year law career raising legal hell, defending controversial clients with an audacity that has antagonized his enemies, including United States intelligence figures and many Jewish groups.