Europe may cross the Rubicon on June 23 when the United Kingdom votes on whether to remain in or leave the European Union. To maintain control of Eurosceptics in his Conservative Party, Prime Minister David Cameron had promised to negotiate for a new deal with the EU and to then offer a referendum, but the referendum could lead to Brexit (British exit from the EU). Last Friday, the negotiations concluded when EU leaders unanimously agreed to a deal. The next four months will test if Cameron’s gamble will pay off or if Europe will take a dangerous step towards ever-greater disunion.

The deal’s wonkish details are not expected to sway many voters to change their mind. Many who wanted to vote “leave” have said that Cameron’s deal isn’t enough, and there is probably no deal which would have been enough. Many who want to vote “remain” would have supported Cameron regardless of how many concessions he brought home from Brussels.

But the deal does bring home some concessions. The UK will not provide EU migrants some benefits until they have stayed in the UK for seven years. The deal recognizes that the UK will have a separate currency. The UK will be better able to deport potential terrorists and criminals, even if they use human rights arguments to stay in Britain. National parliaments can force the EU to drop proposed legislation if 55% of the parliaments object, but in practice this process would be difficult to use.

Open Europe, a British think-tank which The Economist labels as “Eurosceptic”, responded quickly to the deal on Friday night. The co-directors, Raoul Ruparel and Stephen Booth, argued that the deal “is a step in the right direction” and that it “is not transformative, but neither is it trivial.” However, they are disappointed that the EU did not attempt to make Europe more competitive and less bureaucratic. “It is unfortunate that EU leaders have not risen to the occasion and taken this opportunity to embrace Europe-wide reform.”

Many Tories are not satisfied with the deal. Boris Johnson, the Mayor of London who probably has ambition to be the next Prime Minister, is the most notable Conservative Member of Parliament (MP) to abandon Cameron and support Brexit. Yesterday Johnson said, “I don’t think anybody could realistically claim this is fundamental reform of the EU or of Britain’s relationship with the EU.” Because Johnson is the most popular politician in Britain, his defection is a devastating loss for the Prime Minister. The Mayor’s jovial personality will be a major boost for the Leave Campaign and could sway indecisive voters. From my own experience, Johnson is the most charismatic politician I have heard speak in person.

David Cameron argues that remaining in the EU would make Britain safer. As he said Saturday morning in front of 10 Downing Street:

I believe we will be safer in a reformed Europe, because we can work with our European partners to fight cross border crime and terrorism… Let me be clear. Leaving Europe would threaten our economic and our national security.

Brexit supporters will not accept this argument, and on the surface the national security argument is not entirely convincing. The UK will remain part of NATO, which will defend Britain if there is an attack. The UK will continue to have a relatively strong military compared to today’s risks. Remaining in a bureaucratic EU does not clearly add much to national security. Beneath the surface, though, Brexit does pose a risk for Europe’s national security.

The Federalist Papers made the argument that the newly-founded United States should adopt the Constitution to guarantee America’s security. If divided, the states risked either European domination or war amongst themselves. For example, Alexander Hamilton argued in Federalist No. 8:

But if we should be disunited, and the integral parts should either remain separated, or, which is most probable, should be thrown together into two or three confederacies, we should be, in a short course of time, in the predicament of the continental powers of Europe—our liberties would be a prey to the means of defending ourselves against the ambition and jealousy of each other.

Applying this logic to today, Brexit does threaten British national security because it would be a step towards greater European disunion.

Disunity would not happen overnight, but over a decade other countries may seek to leave the EU, especially if the British economy grows faster than the continent’s economy, which it has been doing already. Brexit would be part of a dangerous trajectory with other European events, such as the Greek and refugee crises.

A disunited Europe would not face unmanageable security risks in the short term. Intelligence agencies could share information about possible terrorist attacks with or without a Brussels bureaucracy. NATO would still deter Russian aggression. However, within ten years or so, ever-greater disunion could weaken Europe’s ability to resist its foes. Or some calamity could cause conflict amongst the European countries. These long-term risks are difficult to forecast, but without the EU, Europeans will have fewer tools to tackle unforeseeable threats.

From an American perspective, the US cannot and should not influence the referendum. If the British believe Brexit is the best way to protect their democracy, they should have their vote. However, Americans should carefully monitor Europe’s trajectory towards disunion. The US should consider how its foreign policy may be different after Brexit, even if everything seems normal at first.

In the HBO/BBC miniseries Rome, the pious legionnaire Lucius was appalled when Julius Caesars’ army crossed the Rubicon to overthrow the Senate. As he and his not-so-reverent companion Pullo rode towards an undefended Rome, Lucius said with anguish, “This can only mean that the Republic has fallen.” Pullo responded, “And yet, the sky is still above us and the earth still below. Strange.”

Similarly, the sky will not fall on June 24 if Britain votes for Brexit on June 23. While risks for Britain and Europe would develop slowly over years, life would continue close to normal, at first. Still, Europe would have crossed the Rubicon. The day that Lucius and Pullo rode into Rome appeared calm and normal, but the trajectory towards misadventure was clear.

Mark Melton is the Deputy Editor for Providence. He earned his Master’s degree in International Relations from the University of St. Andrews and has a specialization in civil conflict and European politics. He earned his Bachelor’s degree in Foreign Language & International Trade from Mississippi College. Prior to moving to DC, he worked as a political science adjunct professor at community colleges in Mississippi. He is Providence’s resident millennial (don’t let the premature salt-and-pepper hair fool you, he’s a millennial).

Photo Credit: On February 20, David Cameron walks out of No. 10 Downing Street to deliver a press statement about his EU deal made in Brussels the previous night. By Georgina Coupe for the Crown, via 10 Downing on Flickr.