Wary, not unjustly, of being drawn into a conflict with China over uninhabited rocks, Washington chose not to clarify whether its Mutual Defense Treaty with the Philippines would trigger an American military response.
Beijing sensed Washington's lack of resolve.
So when the US brokered a deal in which the Philippines and China agreed to withdraw their ships, Beijing simply reneged on its part. China kept the blockade intact, which its coastguard enforced by chasing and firing water cannons at approaching Filipino vessels.
For the next four years, Washington failed to craft an effective strategy to get Chinese ships out of the shoal.
Having reopened Scarborough to Filipino vessels, it will be very difficult for Chinese dredgers to undertake land reclamation.
To do so would undercut the political rapprochement that China is trying so hard to foster with the Philippines.
Accordingly, Beijing's purported aim of turning the shoal into a military airstrip within missile range of Manila has, in effect, been put on ice.
China's ability to coerce the Philippines has also been reduced.
Although Beijing can easily reinstate the blockade, this would similarly undercut its political objectives. In other words: the one-off lifting of the blockade has removed an ongoing source of Chinese leverage over Manila.
This suggests Beijing is serious about making actual compromises to improve its ties with Manila at Washington's expense.
Even so, America has sustained considerable geopolitical losses.
The significance of a Chinese compromise at Scarborough is likely to add momentum to political rapprochement between Beijing and Manila.
While more Filipinos view the US favorably than they do China -- 92% to 54% in a Pew surveylast year -- these numbers may well shift in response to Chinese concessions like this.
Such trends could make it easier for Duterte to allay popular concerns about his pro-China foreign policy.
This doesn't mean he will necessarily evict the US military as he's recently threatened. But if cooperation with China continues smoothly, US-Philippines ties could become even more frosty, possible neutralizing the strategic value of a key American ally in Southeast Asia.
More broadly, the way that Duterte, not Washington, brought the four-year Scarborough Shoal standoff to a close exposes a central failure in America's South China Sea policy: Washington's hesitancy to stand up to Beijing lest it spark a military crisis leaves regional partners vulnerable and open to China's entreaties.
The danger is that other countries will follow the Philippines' lead: distancing themselves from the United States and making deals with China that chip away at the regional order. If this comes to pass America's position will be truly weakened.