For decades, US military planning was based on the idea that the US should be able to fight two wars simultaneously in different parts of the world. But even the most pessimistic strategist does not plan three wars at once. However, Joe Biden's administration is currently facing militarized crises in Europe, Asia, and the Middle East. Taken together, they constitute the greatest challenge to American global power since the end of the Cold War. Us officials have briefed that Russia is planning to invade Ukraine "as early as early 2022". Meanwhile, U.S. Defense Secretary Lloyd Austin warned that Chinese military exercises near Taiwan looked like rehearsals for a full-scale invasion. Iran could still be weeks away from producing enough fissile material to build a nuclear weapon -- an outcome the United States has spent decades trying to prevent. Some analysts fear the US may now be facing a concerted global attack by revisionist powers. Carl Bildt, a former Swedish prime minister and international diplomat, warned that policy makers should consider the possibility of invading both Taiwan and Ukraine. "Taken together, these two acts of conquest will fundamentally alter the global balance of power," he said, sounding the death knell for a world order that has "underpinned global peace for decades." The idea of a three-way conference call involving Iranian President Ebrahim Raisi spiraled into pulp fiction territory. But while there is no single plan linking the ambitions of Beijing, Moscow and Tehran, there is a degree of shared analysis and wariness. The Chinese, Russian and Iranian governments have all complained that they have become targets for "regime change" from Washington. They all have ambitions to dominate their own region. Both justify their ambitions by claiming kinship with people beyond their borders. Mr Putin has argued that Ukraine is committing "genocide" against Russian speakers and that Moscow has a duty to protect them. The Iranian government claims to be the protector of Islam around the world and uses Shia Muslims outside its borders as proxies. After its chaotic summer withdrawal from Afghanistan, America looks weak. This increases the temptation for Russia, China and Iran to try to bury old grievances or move forward with long-term ambitions. The revisionist powers of Asia, Europe and the Middle East will each keep an eye on developments on other continents. An unopposed attack on Ukraine or Taiwan would mark the fundamental shift in global power that Mr Bildt and others fear. But US power and credibility may also be diminished by a series of less dramatic conciliatory measures that together suggest the US is retreating. The Biden administration's talk of a new security "accommodation" with Russia in Europe has caused consternation in parts of the region. If the United States backs down in the face of Russian threats over Ukraine, China may be encouraged to step up its intimidation of Taiwan, while Iran may step up its nuclear accelerator. America's Allies, crucial to Washington's global influence, could also become frustrated and start peeling away. The White House is alert to these dangers. It knows it must pick its battles -- or risk becoming dangerously over-committed. A stronger US position in Europe, Asia or the Middle East could help restore US deterrence around the world. But where, if anywhere, should America flex its muscles? The scale of the threat points to China. The scale of provocation points to Russia. On a smaller scale, the risk points to Iran, which is (so far) a non-nuclear power. In strategic terms, the Biden administration's instinct is to cast China as the only possible challenger to the United States as the world's sole superpower. Mr Biden has hinted that the US would defend Taiwan if it were attacked -- but has made no similar comments about Ukraine. Russia's threat to Ukraine is in the shorter fuse. But direct conflict with a nuclear-armed Russia is almost unthinkable. Instead, Biden has threatened to impose massive economic sanctions and military aid on Ukraine in the event of a Russian attack. An air strike on Iran's nuclear facilities looks more dangerous than entanglement with Russian or Chinese forces. But Biden, like Barack Obama and Donald Trump before him, is reluctant to get involved in another war in the Middle East. The Biden administration will not explicitly take the military option off the table in any of the three conflicts. But the US is most likely to rely on economic and diplomatic weapons. Tough economic sanctions of the kind the United States has already imposed on Iran could be used against Russia or China if Ukraine or Taiwan were attacked. This does not mean the start of World War III. But it could spell the end of globalisation.