Today, with the long benefit of hindsight, France's stunning collapse in the face of Nazi invasion looks almost unsurprising. But at the time, it stunned the world. France was one of the preeminent superpowers of the day. It had one of the world's biggest land armies, navies, and second-biggest colonial empire in the world. Moreover, as France had led the Allies in World War I, a war that was orders of magnitude more terrible than anything anyone had ever known, it had a reputation for military invincibility. When in 1923 Germany delayed paying back war reparations, France invaded, occupied, and easily steamrolled the Weimar Republic's puny military.

And this reputation for military invincibility was one of the things that held the world order together. There are countless causes for why the world backslid into World War II, but an underrated one was the sense that if Hitler really got out of hand, the French and the British together would crush him.

Today, global peace rests on many things, but one of them is the assumption that the United States military is invincible. We justly fill our headlines with reports of casualties in Iraq, Syria, and Afghanistan, but what is striking in our current era is just how little conflict there is. And one reason for that is that no contemporary military can hope to match the United States', so countries that might want to mess with the U.S. or its allies either don't, or do so through comparatively much less destructive and unconventional means, like hacking.

But just like France's invincibility on the eve of World War II, America's military invincibility may just be waiting to be toppled by anyone clever and gutsy enough to give the right shove.

Here are three very worrying ways in which America's conventional war machine is being outclassed.

1. Supercarriers

He who rules the seas rules the world. It was true in the time of the Greeks, and it's true today. And on paper, America's dominance looks total. The United States has 10 aircraft carriers. Russia can barely field just one. China only just recently got one, a retrofitted old Soviet clunker. And in some way, this undersells America's advantage: America produces supercarriers which are on the order of twice as large as anything else on the sea, and nuclear-powered, which means they can stay at sea much longer (the only other country with a nuclear carrier is France). Carriers are the dominant means of "force projection" (translation: going out and kicking someone's ass), and have been since World War II, when they and their planes proved much more destructive than the old battleships.

But here's the thing: Just like France's outdated tactics were obsoleted by German Blitzkrieg, carrier strike groups, a technology and formation from the mid-20th century, are probably obsolete. As an excellent article by David W. Wise convincingly argues, aircraft carriers are probably extremely vulnerable to a number of new technologies, from asymmetric warfare to super-quiet submarines to advanced ballistic missiles. In military exercises, U.S. aircraft carriers keep getting sunk.

Up until very recently, America's overwhelming carrier advantage meant that any attempt, say, by China to invade Taiwan, looked like folly. Now it practically looks like an invitation: With its anti-ship ballistic missiles, China could sink half the U.S. Navy before it even got within range of the island.

It increasingly looks like the Navy of the future will mostly consist of drone- and missile-launching submarines (manned and unmanned), which hold a number of decisive advantages over carriers. But these are areas in which the Navy, despite some interesting experiments, is under-investing — partly because its budget is being eaten up by a frenzy to build and maintain ever-more expensive supercarriers.

2. Stealth fighters

Like naval power, air power is absolutely crucial in war. He who controls the skies controls the fight. Observers and historians often joke that Israel's Six-Day War should really be called the One-Day War; Israel was able to crush vastly superior enemies on two fronts at once because it destroyed their air forces in a masterful preemptive strike, making the rest of the war a formality. Every single conventional military victory by the United States since the end of the Cold War has been premised on, and enabled by, total dominance of the skies. So making sure that, in any conventional war, the United States can establish and maintain air dominance is front and center for all the strategic planners at the Pentagon.

Thankfully, they have a silver bullet: stealth! All of the United States' fighter jets will be stealthy. And when you can't even show up on the enemies' radar screens and you can shoot at them with impunity, you're going to crush them very quickly, right? Billions and billions (and billions) were poured into projects such as the F-35 and F-22 (and crucial design tradeoffs were made) so that those planes could have "stealth technology."

But there are two problems. One is that Russia and China are also building their own stealth fighters. And the other is that stealth technology isn't actually all that stealthy. In a development that will shock only those who don't know anything about the history of warfare, America's competitors have come up with technologies to counter these gizmos. In fact, a number of new technologies, including active and passive radars, are pretty good at detecting so-called "stealth" planes.

The U.S. Air Force's strategic objective is to have enough of an edge, whether technological or organizational, that it can easily crush a rival's air force. The method for doing so since the early 1990s has been "Stealth! Stealth!" But now, we need something new.

3. Networks and satellites

Since the mid-1990s, strategic planners in the Pentagon have boasted that so-called "network-centric warfare" has revolutionized military affairs. Everyone in the battlefield will be networked with everyone else, thus removing the "fog of war." Commanders will know instantly where everything is, and troops will be able to respond to threats much more quickly and efficiently.

Since 2003, the United States has tried a bit of network-centric warfare of its own. And while the idea of using technology to enable various elements on the battlefield to communicate more efficiently and to enable commanders to have better information is tautologically sound, it isn't the gamechanger so many in the Pentagon seem to think it is. A network-enabled force is great — if your networks work. During the 2003 invasion of Iraq, "network-centric warfare" did enable some tactical advantages, but there were also a lot of bugs. Maybe those kinks are to be expected. Or maybe this is an example of the tendency of the peacetime Pentagon contracting process to produce enormously expensive, ambitious technology that just doesn't work. The Pentagon's fabulous war-enabling network could crash during an encounter with China over Taiwan or Russia over the Baltics at the worst possible moment.

On top of that, an information network has to route all of its data through somewhere. And for the United States, that somewhere is satellites, which makes some sense, since the U.S. has the best satellite technology in the world. Here's the problem: Satellites are enormously vulnerable. Anti-satellite missiles are a thing. Why wouldn't the Russians knock down the United States' network in the first move of an engagement, leaving its military deaf, blind, and dumb?

If I'm right about any of this, it means that the United States military is glaringly vulnerable to a devastating attack in a way disturbingly similar to the heretofore-invincible 1940 French military. And if I know this, I'm sure there are people in Moscow and Beijing who do, too.