Zonath is right in the gist of his post but wrong on several details.
First, "a substantial risk of confusion" is not the test for infringement. It's "likelihood of confusion."
Second, I'm not sure where Zonath gets the "marketed" in a foreign country bit. 15 U.S.C. §1111 doesn't mention that word or concept. The TMEP does refer to "foreign applicants" and their use of the circle-R when marks are registered in foreign countries, but there's no mention of "marketed" there either.
Finally, the whole bit about "you would need to license" the mark may be wrong. Whether or not you would need to do that depends on a lot of factors, including the similarity of your "mark" to the other party's, and the nature of your use, to mention a couple.