Macquarie's head of FX and rates strategy, Gareth Berry, has noted that the "demonstration effect" of the coordinated U.S.-Japan intervention on the yen carries "far more shock value than the size of the intervention itself."

In a recent report, Berry wrote, "We estimate U.S. authorities sold roughly $500 million worth of EUR/JPY on Friday, July 31st."

"By comparison, this is merely a drop in the bucket against Japan's estimated $85 billion in USD/JPY selling over the two days of July 30th and 31st."

Given the relatively modest sum the U.S. deployed in last month's intervention, it's clear that "ample firepower remains at the ready."

"Combined, the U.S. Treasury and the Federal Reserve alone can tap into as much as $25.9 billion in euro-denominated reserves. So, if the yen were to weaken again, they are fully equipped to intervene in EUR/JPY on a similar scale and multiple times."

Theoretically, if the U.S. were to shift its focus to intervening directly in USD/JPY, its ammunition would be virtually limitless.