If only one country signs the deal it is not going to be very effective. The US in particular is highly likely to already have other deals in place already. The real question is how to make it global or something close to it.
How about instead the target being a conference, meeting or gathering were Earth representatives convene to discuss signing the deals?
Another good one is who is already willingly allied to the authoritative ETs besides the US and how much of the US military is not since there seems to be factions in favor and against it.