Over the next 15-20 years, the U.S. and China are headed for a confrontation in the western Pacific, with Japan caught in the middle. And China, currently the underdog, could very well come out on top. That's the unnerving conclusion of a new report by the Carnegie Endowment for International Peace, a Washington, D.C.-based think tank.

The nine authors of "China's Military & the U.S.-Japan Alliance in 2030," released Thursday, stress that a full-blown shooting war is not in the cards. "The threat is not a war with China," the report states. Rather, Washington and its close ally Tokyo could find themselves losing influence and disputed island territory to an increasingly unconstrained Beijing that finds it can "win without fighting" owing to a combination of its own military rise and its rivals' relative declines.

But Chinese "victory" in this projected 2030 conflict is not preordained. It's also feasible the U.S. and Japan could "win" as their own armies and economies rally against a China dragged down by shrinking exports and demographic stagnation.

In any event, change of some sort is probably coming, the report authors say, although what change is unclear. The status quo – a western Pacific comfortably dominated by the U.S. with its aircraft carriers, bombers and Marine regiments, with Japan playing a key supporting role and China steadily adding to its military arsenal while biding its time – is "unsustainable," they claim.

What follows are sketches of three possible scenarioses from the report, representing two extremes plus a sort of strategic middle ground in the anticipated Great China-America Showdown of 2030.

New World Order

China's 10-percent annual economic growth continues unabated despite high debt, an aging population and vexing ecological concerns. The People's Liberation Army enjoys year after year of elevated spending. Its homegrown ships, planes and missiles get better and better alongside improving Chinese military doctrine, leadership and training.

But on the opposite side of the Pacific, the United States succumbs to its own internal problems. Economic growth slows to just 1.5 percent per year, leading to what the Carnegie experts describe as "enormous downward pressures on U.S. defense spending and U.S. military deployments in Asia."

The stealthy F-35 Joint Strike Fighter, meant to rescue U.S. air power from obsolescence, instead falters, as does an ambitious program to develop a new, affordable stealth bomber. A shrinking American naval fleet runs low on floating sonobuoys used to detect the new and improved Chinese submarines pouring out of the country's shipyards.

Broke and demoralized, America retreats from the Pacific, leaving an equally struggling Japan to fend for itself against its powerful neighbor. Instead of ramping up its own military spending in order to confront China, Japan strikes a conciliatory tone with the world's new Pacific hegemony.

This worst-case (for Washington and Tokyo) combination of events is "highly unlikely but not entirely inconceivable," according to the Carnegie report. But if they or similarly bleak circumstances come to pass, the Asia-Pacific in 2030 truly will belong to Beijing. "Needless to say, this scenario would present an enormous potential for severe crises."

America Rules

The opposite extreme is an era of substantially increased U.S. presence in the western Pacific. For Washington and Tokyo, a stronger Asia-Pacific posture would result in "a more stable long-term regional security environment," according to the Carnegie authors. Especially if American resurgence meets with a collapse in Chinese military capabilities and strategy.

But this rosy scenario counts on a big boost in U.S. military spending, amounting to a full four percent of GDP – an assumption that itself hinges on a rapid economic recovery from today's depressed levels plus a sustained high birth rate. In addition to more spending, the "America-wins" future assumes ideal outcomes across a wide range of military initiatives. The F-35 fighter and the new stealth bomber come in on time and on budget – and they both work as advertised. A U.S. plan to expand its naval fleet also proceeds without a hitch.

At the same time, all the major trendlines towards a stronger and more assertive Chinese military would have to reverse themselves – and fast. The open spigot of weapons funding would have to close. Technologies already in development would have to fail. Training exercises currently growing more realistic would need to end or somehow get dumber.

In short, this outcome is unlikely. "It is quite probable that the United States and Japan will lack the financial resources, technological capacity and political willpower necessary for such an ambitious military response, especially in the next 15 to 20 years," the report warns.

Middle Ground

So which scenario is probable? America does not collapse back into economic crisis but neither does yearly growth reach the three-percent threshold policymakers desperately hope for. China also enjoys moderate economic expansion sustaining continued improvement of its air, sea, land, space and cyber forces.

Japan meanwhile navigates perilous domestic politics in order to somewhat increase its own military investment, resulting in a more powerful navy, a small number of stealth fighters for its air force and a naval infantry force modeled on the U.S. Marine Corps.

This "slightly unstable" scenario is the "most likely" of those studied, according to the experts. With relations between China and the U.S.-Japan alliance slowly eroding year on year, "this situation would result in a greater likelihood of tensions and incidents" compared to now. But the chance of major flare-ups, to say nothing of a shooting war, would be as remote as it is today.

This version of the western Pacific circa 2030 is also probably the best that anyone in Washington should hope for, given economic, cultural and strategic realities. It's "manageable," and its military balance still "slightly" favors the U.S, according to the report.

If the Carnegie experts are to be believed, the likely future of U.S.-Japanese-Chinese relations looks a lot like today, although more volatile. In that sense the think tank's report could be mistaken for, well, accepted wisdom. It's tempting to project current, short-term trends in straight lines over decades, although in reality today's trends are often fleeting – and poor predictors of the future.

It should not come as a surprise, then, if the seemingly unlikely fringe scenarioses spelled out in the Carnegie report – both pro-U.S. and pro-China – look a lot more realistic in just a few short years. Between America and China, with Japan watching closely from the sidelines in the world's new strategic center of gravity, the future could be a toss-up.