"Americans are from Mars and Europeans are from Venus." With one pithy line 14 years ago, foreign policy theorist Robert Kagan captured what seemed like obvious differences between the United States and Europe in the post-Cold War, post-9/11 age.
Most Europeans, Dr Kagan argued, thought of themselves as citizens of "a post-historical paradise of peace and relative prosperity", in which threats could be safely managed without military force, and systems of "transnational negotiation and cooperation" were making war and nationalism obsolete.
Most Americans, on the other hand, still saw geopolitics through a Hobbesian lens: as a struggle for mastery in which threats need to be confronted quickly, multilateralism is at best a luxury, and liberal values flourish only under the umbrella of overwhelming military might.
This divide, Dr Kagan suggested, flowed naturally from power differentials. Because the US was uniquely powerful, Americans still had great faith in action and resolve. Because European powers were far weaker than in the past, they preferred to believe in a world system where national strength would count for less.
Dr Kagan's argument appeared amid the Iraq War debate, which seemed to perfectly illustrate his premise. Even if (as he conceded) the Mars and Venus language oversimplified things, anyone who observed Western politics in that era could see the pattern he was talking about, the assumptions that separated the US perspective from the world view of Brussels, Paris and Berlin. Which is why, in this time of political turmoil on both sides of the Atlantic, it's so striking to watch Mars and Venus reverse their Bush-era alignment.
It's too soon to say Europeans are actually from Mars once again. But the continent's Venusian idyll has taken blow after blow: the euro crisis, the aggressions of Russian President Vladimir Putin, and now the convergence of mass migration and Islamist terror. Nationalism is returning, border fences are going up. The centre is weakening, the far right is gaining power. The Mediterranean and the Russian marches are zones of conflict again, and ancient habits - French military adventurism, Little Englander separatism, a tense relationship with Islam - are resurfacing.
Because the United States was uniquely powerful, Americans still had great faith in action and resolve. Because European powers were far weaker than in the past, they preferred to believe in a world system where national strength would count for less.
The European elite still believes in the Kantian dream of perpetual peace, which is how the continent ended up with German Chancellor Angela Merkel's open-door policy for Syrian refugees. But its leaders are also adapting to post-Kantian reality, and nowhere more so than in France, where the government has basically gone Le Bush-Cheney under both Mr Nicolas Sarkozy and Mr Francois Hollande: intervening in Libya, Mali and Syria; responding to terrorist attacks with Bush-esque rhetoric; and implementing a terror crackdown that makes the Patriot Act look libertarian.
In certain ways, America is mirroring these trends: We're involved in Libya and Syria as well, we have our own refugee-related anxieties, and in Republican presidential hopeful Donald Trump, we have our own version of Europe's nationalist right. But if nationalism is making Europeans more militaristic, in America, it's inclining us to lay down the burdens of empire, to retreat into a self-sufficient Arcadia all our own.
That's a subtext of Mr Trump's rhetoric. Making America great again involves crushing the Islamic State in Iraq and Syria, yes, but otherwise it seems to involve washing our hands of military commitments - ceding living space to Mr Putin, letting Japan and South Korea go nuclear, calling Nato obsolete. And it's simply the text of Democratic presidential candidate Bernie Sanders' campaign. He's running explicitly as the candidate of Venus (or Scandinavia, if you prefer), promising socialism at home and an end to military adventures abroad.
For now, our elites are still Martian. A President Hillary Clinton will probably have more in common with Mr George W. Bush on foreign policy than she does with either Mr Trump or Mr Sanders. But over the longer run, in a more fractured country and a more chaotic world, the desire for splendid isolation may only increase. There's no mass constituency for liberal hawkishness in the Democratic Party anymore. The ease with which Mr Trump dispatched rivals Jeb Bush and Marco Rubio suggests that neoconservatism, too, is vulnerable to a "come home, America" message.
What's interesting, and somewhat in tension with Dr Kagan's original argument, is that all of this is happening without a major change in the relative power of the US and Europe. America is still the only hyperpower; they're still militarily weak. France and Britain couldn't have toppled Libyan dictator Muammar Gaddafi without us. The nations of Eastern Europe still need our protection against Mr Putin.
This unstable combination suggests that the transatlantic relationship may be headed for a strange inflection point, a kind of through-the-looking-glass version of the Iraq debate.
The time is the late 2020s, let's say, and the French and Germans and Poles demand that the US lend our still-unparalleled military strength to a conflict that seems essential to European security - toppling a nascent caliphate in North Africa, or recovering weapons of mass destruction from a collapsing post-Putin Russia.
And a Socialist administration in Washington, backed by more than a few Trumpian Nationalists in Congress, looks across the ocean at Europe's wars and whispers, "not this time".
NEW YORK TIMES