Adam Clarke on 1 Samuel 11:1

Nahash the Ammonite - In the Vulgate this chapter begins thus: Et factum est quasi post mensem, "And it came to pass about a month after." This addition appears also in the principal copies of the Septuagint; though it is wanting in the Complutensian edition, both in the Greek and Latin, and is not acknowledged by any of the Oriental versions. But it is in Josephus, and probably was inserted from him into some copies of the Septuagint, and thence into the Vulgate. It appears to be of very little authority.

We know little about Nahash; there was a king of this name among the Ammonites in the time of David, 2Sam 10:2, but probably not the same person. Nahash might have been a common name of the Ammonitish kings.

Make a covenant with us - They found they were in no condition to risk a war; and they wish to have peace, and desire to know his conditions.