This lawsuit is a legal mess, destined to fail. In fact, it already did fail and they're just trying futilely to revive it. All applicable statutes of limitations passed years ago. You can't wait decades to file a lawsuit. Equally as importantly, "The Marshall Islands" as a political subdivision does not have standing to sue for injuries that occurred to specific people and property there. Those people and property owners would have to sue, not their regional government. Finally, the decisions which were made and the actions taken were political decisions made by the United States in exercise of its sovereign authority - and you can't sue for that. It seems that the plaintiffs know this, which is why they are now trying to frame the lawsuit as a claim to enforce the NPT. The problem with that is, yet again, a lack of standing on several levels, and an inaccurate interpretation of the treaty itself. First, there is no cause of action through which any individual or entity can force the government to comply with or enforce a treaty. International relations are solely the sovereign domain of the federal government, and they can decide to abide by (or disregard) treaties as our elected officials see fit. Second, the treaty is not being violated. It does not require disarmament, nor is there a mandatory timeline for any particular disarmament-related activity. It says the signatories will negotiate towards an agreement regarding disarmament. That's not an enforceable mandate in any meaningful sense. Why? Because the signatories never actually had any intention of disarming, so they made an agreement that didn't require them to disarm. A third party can't come in and force them to abide by a deal they didn't make in the first place.
Look, the Marshallese got screwed. There was a discriminatory component to that. It wouldn't happen the same way today. But the bottom line is that we needed a place to test weapons of mass destruction, and the Marshall Islands were the best choice available. So the US did what they had to do to make the program work. They should have provided market-based compensation for the taking of land, and they should have relocated everyone out of the zone of danger, turning the entire area into a restricted military installation before blowing it up repeatedly. There should have been no injuries and no uncompensated loss of property. But the reasonable conclusion to take away from those events is not that nuclear weapons should be eliminated, or that the tests shouldn't have been conducted at that location. They served a critical purpose for national security, and anyone who says otherwise is a revisionist with an agenda.