In 1784, as the representative of the Creek and Seminole nations, he formed a treaty of alliance with Spain, by the terms of which he became a Spanish commissary with the rank and pay of a colonel.
Against the State of Georgia, the Creek nation had grievances which McGillivray was able to voice with a vigor and an eloquence that compelled attention.

It was the old story, so often repeated in American history, of encroachments upon Indian territory.

Attempts at negotiation had been made by the old government, and these were now renewed by Washington with no better result.

McGillivray met the commissioners, but left on finding that they had no intention of restoring the Indian lands that had been taken.

A formidable Indian war seemed imminent, but Washington, whose own frontier experience made him well versed in Indian affairs, judged correctly that the way to handle the situation was to induce McGillivray to come to New York, though, as he noted in his diary, the matter must be so managed that the "government might not appear to be an agent in it, or suffer in its dignity if the attempt to get him here should not succeed." With his habitual caution, Washington considered the point whether he could send out an agent without consulting the Senate on the appointment, and he instructed General Knox "to take the opinion of the Chief Justice of the United States and the Secretary of the Treasury." The assurances obtained were such that Washington selected an experienced frontier commander, Colonel Marinus Willett of New York, and impressed upon him the importance of bringing the Indian chiefs to New York, pointing out "the arguments justifiable for him to use to effect this, with such lures as respected McGillivray personally, and might be held out to them." Colonel Willett was altogether successful, though the inducements he offered were probably aided by McGillivray's desire to visit New York and meet General Washington.