The Christchurch Call was seen as considerably weaker when the White House snubbed it two years ago.
The Call is a coalition of 51 countries and organisations and 12 online platforms to tackle terrorist and violent extremist content online. It followed the March 15 terrorist attack in Christchurch, which was livestreamed in a video uploaded hundreds of thousands of times in 24 hours.
The US left a big hole in the coalition, and was never considered likely to join because of how zealously it guards free speech rights.
The Trump Administration, in 2019, instead offered a "hurrah" from the sidelines in the form of a public statement applauding the intent, which was seen as a significant step in itself.
So it was a genuine surprise to see a change of heart - perhaps a natural one, given the change in administration - from the US yesterday in announcing it will join.
On the surface, the US State Department seemed to downplay the move. It said the US will not do anything to violate the right to free speech, free association, or privacy rights. It even warned other governments and tech companies not to use the Call to restrict freedom of expression.
This looks like enough caveats to make its sign-up redundant, but these points simply echo what the Call's manifesto already says.
The real test will be how much heft the US brings to the table - the measure of any impact in a voluntary framework.
It could make a tangible difference in several ways.
More political will behind the US's involvement with the independent GIFCT (Global Internet Forum to Counter Terrorism) could turbo-boost research into, for instance, preventing March 15-like videos from being uploaded in the first place.
Rather than being on the periphery, the US could become heavily involved in the global, collaborative response to minimise spread when such videos are shared online. This formalised response is perhaps the biggest practical difference the Call has made after the Christchurch mosque shootings, when "what's everyone doing?" calls from the Beehive to tech companies exposed how inadequate the system was.
The crisis response has already been used, and the addition of the US will likely bolster continuous improvement; the response to the 2019 terrorist attack in Halle, Germany, was an improvement on the March 15 response, but communication between Call partners could have still been more rapid.
And, as signalled by the US State Department, it intends to focus on the ability of online platforms to divert users away from potentially harmful content.
These are the online rabbit holes - from the likes of Facebook and YouTube - that can draw users ever-deeper down the hole towards increasingly more extreme content.
A push from the US in this area will be welcomed by Prime Minister Jacinda Ardern, who has been particularly interested in how users - however unintentionally - can be radicalised by the content thrown up by certain platforms' algorithms.
According to a recent report, only 9 per cent of those in the Christchurch Call community wanted the algorithm work to be a priority. US influence could see this work given renewed impetus.
Any changes to how online platforms operate will still come down to the tech companies themselves, and they have been resistant to anything that loosens their grip on users' attention.
In any case, the move by the US is symbolically a big deal, regardless of how much noise it will make. The superpower is influential, adds a weight of experience, and is the home base for the major online platforms.
Much better to have such a player in the room than applauding from the sidelines.