The move by Washington's close ally set off an avalanche. Irked that London had stolen a march, Germany, France and Italy announced that they too would participate. Luxembourg and Switzerland quickly followed suit.
The trail of transatlantic and intra-European diplomatic exchanges points to fumbling, mixed signals and tactical differences rather than to any grand plan by Europe to tilt to Asia.
That is nevertheless the way it is seen by some in Washington and Beijing.
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As recounted to Reuters by officials in Europe, the United States and China who spoke on condition of anonymity because of the sensitivity of the subject, the episode reveals the paucity of strategic dialogue among what used to be called "the West".
It also highlights how the main European Union powers sideline their common foreign and security policy when national commercial interests are at stake.
China's official Xinhua news agency reflected Beijing's delight.
"The joining of Germany, France, Italy as well as Britain, the AIIB's maiden G7 member and a seasoned ally, has opened a decisive crack in the anti-AIIB front forged by America," it said in a commentary.
"Sour grapes over the AIIB makes America look isolated and hypocritical," it said.
Of the main U.S. allies in Asia, Australia appears close to joining, though no formal decision has been made, and Japan and South Korea are considering the possibility.
"The Americans are starting to look very mean-spirited with their criticism," said a Beijing-based Asian diplomat. "This is not a battle they are winning. Even their closest allies in Asia are starting to fall in line."
Anger at stalled IMF reform
In Europe as in Washington, China's launch of a new institution to channel a fraction of its massive currency reserves into infrastructure investments in Asia posed a political conundrum and provoked turf disputes.
Western countries had long urged Beijing to recycle some of its trade surplus into building transport, energy and telecommunications networks in developing nations, but they wanted it to use the World Bank and the Asian Development Bank, dominated by the United States and Japan.
China, angered that the U.S. Congress has not ratified a 2010 agreement to increase its voting share and that of other emerging economies in the International Monetary Fund, chose to go its own way instead.
With initial capital of $50 billion, the Beijing-based AIIB can offer at most a complement to the larger World Bank and ADB, but it is starting point for expanding Chinese influence.
Officially, the United States says it is concerned about whether the bank will uphold human rights, environment and labor standards and be open and transparent in its governance.
In private, senior U.S. officials acknowledge this is about power. One Obama administration member said Congressional foot-dragging on IMF reform had "created an opportunity forChina to assert itself".
Lew gave a blunt assessment last week, telling U.S. lawmakers: "It's not an accident that emerging economies are looking at other places because they are frustrated that, frankly, the United States has stalled a very mild and reasonable set of reforms in the IMF."
Republican Senator Jeff Sessions of Alabama acknowledged irritation about IMF voting rights may have been a factor.
"I think this could be an unfortunate event and it might be bigger than we understand today," he told the Brussels Forum, an annual transatlantic dialogue organized by the German Marshall Fund of the United States.
In Washington, the issue resided between the State Department, the Treasury and the White House National Security Council, which may have muddied U.S. communication with European allies, officials say.
"There just wasn't a clear and coherent and unified message on this from the beginning. It kind of languished for a while in a state of indecision and that produced the outcome that you've seen," said a Congressional source familiar with the discussions.
Within European governments there were debates about tactics and timing but the prevailing view was that it was better to try to influence the Chinese project from inside, several officials said.