Update: According to reports in on Monday evening, the USS Lassen has indeed sailed within 12 nautical miles of China's islands in the Spratlys.

As WSJ notes, "an American defense official confirmed Monday that the U.S. Navy ship navigated through the waters around at least one of the land masses to which China lays claim within the Spratly chain of islands in the South China Sea, crossing an area that China maintains is part of its sovereign territory."

WSJ also reiterates that this isn't likely to be a one-off event. As noted below, most "experts" believe that in order for this to be effective from a deterrence standpoint, the US will need to step up the patrols, presumably in an effort to prove to Beijing that the Pentagon is "serious", whatever that means in this context.

The ball is now squarely in China's court. The PLA has already promised to "stand up and use force" in the event its territorial sovereignty is violated. The question now is whether Beijing will back down and concede that "sovereignty" somehow means something different with regard to the islands than it does with respect to the mainland or whether Xi will stick to his guns (no pun intended) and take a pot shot at a US destroyer.

Earlier:

For anyone who might still be somehow unaware, the US is currently in a superpower staring match with both Russia and China. The conflict in Syria has put Moscow back on the geopolitical map (so to speak), creating an enormous amount of tension with Washington whose regional allies have been left to look on in horror as Russian airstrikes and an Iranian ground incursion dash hopes of ousting President Bashar al-Assad.

Meanwhile, in The South China Sea, Beijing has built 3,000 acres of new sovereign territory atop reefs in the Spratlys and although the reclamation effort itself isn’t unique, the scope of it most certainly is and Washington’s friends in the South Pacific are crying foul.

Beijing has continually insisted that it doesn’t intend to use the islands as military outposts, but the construction of runways and ports seems to tell a different story and so, Washington felt compelled to check things out over the summer by sending a Poseidon spy plane complete with a CNN crew to the area. Once the PLA spotted the plane the situation escalated quickly with the Chinese Navy telling US pilots to “Go Now!”

After that, an intense war of words developed with Defense Secretary Ash Carter insisting that the US would sail and fly anywhere it pleased and Beijing assuring the US that sailing within 12 nautical miles of the islands would prompt a harsh response from the PLA.

For weeks, the US was rumored to have been planning a freedom of navigation exercise in the Spratlys which, as we’ve pointed out several times this month, amounts to sailing by the islands just to see if China will shoot.

Now, according to CNN, Obama has given the green light and the ships may sail within 24 hours:

Just in: @USNavy prepared to sail w/i 12mi of China's manmade islands in #SouthChinaSea "w/i 24hrs", has POTUS approval -Def. Official — Jim Sciutto (@jimsciutto) October 26, 2015

And more from FT:

The US navy is poised to start freedom of navigation operations in the South China Sea in a high-stakes effort to push back against Chinese territorial claims over artificial islands in the disputed waters. In a move that will enrage Beijing, the USS Lassen, a guided-missile destroyer, will sail inside the 12-nautical mile zones of two man-made islands — Subi and Mischief reefs — that China has built in the contested Spratly Island chain. A senior US defence official said it would sail through the area in the early hours of Tuesday morning. China has repeatedly warned that it would not tolerate any effort to violate what it considers its territory. Earlier this month, a senior Chinese naval officer said the People’s Liberation Army would hand a “head-on blow” to any foreign forces that violated Chinese sovereignty. His comments came after the Financial Times reported that the US was poised to launch its operations. The manoeuvre will mark the first time since 2012 that the US navy has sailed through the 12-nautical mile zone surrounding any islands claimed by China. It is aimed at demonstrating that Washington does not recognise any territorial claims over artificial islands in the South China Sea.

It's also worth noting that should the US manage to get away with this without sparking a shooting war with the Chinese, it now looks as though Washington is leaning toward making this a regular patrol. Here's a bit of color from Reuters out over the weekend:

A range of security experts said Washington's so-called freedom of navigation patrols would have to be regular to be effective, given Chinese ambitions to project power deep into maritime Southeast Asia and beyond. "This cannot be a one-off," said Ian Storey, a South China Sea expert at Singapore's Institute of Southeast Asian Studies. "The U.S. navy will have to conduct these kinds of patrols on a regular basis to reinforce their message." But China would likely resist attempts to make such U.S. actions routine, some said, raising the political and military stakes. China's navy could for example try to block or attempt to surround U.S. vessels, they said, risking an escalation.

(USS Lassen)

Here are the latest visuals from Subi and Mischief (the two islands mentioned above):

Not to put too fine a point on it, but this borders on the insane. Here we have both Washington and Beijing risking an outright military confrontation over what amount to a couple of sandcastles and while there's probably some truth to the contention that China has plans for the islands that go beyond growing plants, building lighthouses, and raising pigs, it's not as though the PLA is going to invade The Philippines so at the end of the day, this looks like another example of what Vladimir Putin recently suggested is evidence that the world is losing its collective mind.