Tuesday night's GOP debate was immensely entertaining political theater, but mostly devoid of policy substance. There was, however, one real exception: the debate over Syria and Bashar al-Assad.

This began when the moderators asked Ted Cruz about his belief that the US should have left Iraq's Saddam Hussein and Libya's Muammar Qaddafi in power. His answer — dictators can be America's friends, especially in Syria — kicked off a debate that pitted Cruz against Marco Rubio, with Donald Trump and Rand Paul on Team Cruz while John Kasich and Jeb Bush stood with Rubio.

The ensuing debate revealed one of the most interesting — and important — foreign policy divides in the Republican Party today.

Is America better off with Assad or without him?

Cruz took what he called an "America first" position on dictatorships generally and on Assad specifically. The US, he said, should consider its own interests first. According to Cruz, America's primary interest in Syria is defeating ISIS. Assad could help with that mission, and toppling him would set it back; therefore, the US should not try to push out the Syrian dictator.

CRUZ: If we topple Assad, ISIS will take over Syria and it will worsen national security interests. And the approach — instead of [being] a democracy promoter, we ought to hunt down the enemies and kill ISIS rather than creating opportunities for ISIS to control new countries.

It's a pretty classic realist foreign policy position: We need to ally with a bad guy in order to defeat the even worse guys.

Rubio, who has argued in the past that the US should help Syrian rebels overthrow Assad, took the position that Assad was still a bad guy who was bad for American interests and should be treated as such. He drew a distinction between friendly dictatorships we ought to work with, like those in Jordan and Saudi Arabia, and anti-American ones, like the Iran-aligned Assad, who should be opposed:

RUBIO: The government in Saudi Arabia is not a democracy but we have to work with them. The government in Jordan is not perfect but we have to work with them. But Assad, who let them get IEDs into Iraq: if he goes, I will not shed a tear.

John Kasich took Rubio's side, arguing that "there are moderates in Syria who we should be supporting," and that "we can't back off of this."

Then something interesting happened: It became a debate about the 2003 US-led invasion Iraq, showing that this was in many ways a disagreement about more than just Assad specifically. Trump, the clear frontrunner, agreed with Cruz that the US needs to stop spending money on campaigns to topple dictators like Assad:

We have spent $4 trillion trying to topple various people that, frankly, if they were there and if we could have spent that $4 trillion in the United States to fix our roads, our bridges and all of the other problems, our airports and all the other problems — we have we would have been a lot better off. I can tell you that right now. We have done a tremendous disservice not only to the Middle East but to humanity, the people that have been killed, the people that have been wiped away and for what? It's not like we had victory. It's a mess. The Middle East is totally destabilized, a total and complete mess. I wish we had the $4 trillion or $5 trillion.

Jeb Bush stood up for his brother's war, saying we're better off without Saddam Hussein. Rand Paul, ever the non-interventionist, opposed "regime change" interventions basically categorically.

The debate over toppling Assad, then, seemed to divide the Republican candidates both over Syria and over larger and more ideological questions.

What this debate means

For most of the campaign, Republicans broadly agreed on foreign policy issues: The Iran deal was a disaster, we should crush ISIS, Vladimir Putin is emboldened by Obama's weakness, and so on. Rand Paul was at times an exception, but he had little shot at the nomination. There had been a long-simmering disagreement on the issue of Assad, but it has never really been a focus of the race.

But last night's debate brought that disagreement into the open, and showed that it genuinely splits the field. Perhaps more importantly, it pitted the two men leading in the polls — Donald Trump and Ted Cruz — against their most plausible establishment challenger, Marco Rubio. So the stakes here are real.

If elected, Rubio would pursue a policy — support the Syrian rebels in a campaign to topple Assad — that would represent a significant shift from the current US policy, which does not openly support any campaigns to topple Assad. Likewise, Cruz's policy — destroy ISIS and leave Assad in power — differs from the Obama administration's current policy of assisting ISIS's moderate rebels while attempting to negotiate a peace settlement that would (ideally) usher out Assad. Trump is less clear on policy specifics but is clearly aligned with something like Cruz's approach.

The Cruz/Trump/Paul position is a challenge to what has been Republican establishment conventional wisdom since George W. Bush and particularly since 9/11: the idea that the United States should use its military and political might to promote democracy abroad. Dictatorships, the idea went, were the root cause of groups like ISIS and al-Qaeda: You install democracy in the Middle East, and these groups will collapse.

These disagreements opened up a bit with the 2011 Arab Spring protests. Some Americans greeted these movements as great news, whereas others saw them as dangerous threats to the status quo and to reliable if unsavory Middle Eastern dictators. When the revolutions turned sour — civil wars in Syria and Libya, the rise of the Muslim Brotherhood in Egypt, the rise of an Iranian-aligned militia in Yemen — US opinions began to shift.

You can see this divide, for example, in how Republicans have responded to Abdel Fattah el-Sisi, the Egyptian military dictator who crushed that country's nascent democracy. Some, such as Rubio and Sen. John McCain, criticize both Sisi and the Obama administration's support for him. Others, such as Cruz, have embraced Sisi as an important ally against Islamism.

This goes beyond Egypt or Syria, though, to the point of general principle. As Cruz put it in an address last week to the Heritage Foundation:

We will not win by replacing dictators, as unpleasant as they may be, with terrorists who want to kill us… we are not abandoning freedom, we are doing what we must to protect it, because the true threat to the spread of liberty is the radical Islamism that is every bit as oppressive as Soviet communism.

Rubio, by contrast, represents the old-guard Bush position that the US should attempt to promote democracy wherever possible. From a May speech:

In recent years, the ideals that have long formed the backbone of American foreign policy—a passionate defense of human rights, the strong support of democratic principles and the protection of the sovereignty of our allies—these values have been replaced by at best caution, and at worst, an outright willingness to betray those values for the expediency of negotiations with repressive regimes. This is not just morally wrong. It is contrary to our interests. Because wherever freedom and human rights spread, partners for our nation are born. But whenever foreign policy comes unhinged from its moral purpose, it weakens global stability and forms cracks in our national resolve.

So this debate isn't just about Syria. It speaks to a fundamental difference in vision between the leading GOP candidates for president on the use of American power. Given that presidential candidates tend to set the tone for their party on foreign policy (at least for the duration of the election and maybe longer), this really matters.