Negotiations over a potential agreement about Iran’s nuclear program are coming to a head this week, with a new deadline of July 7 set for the outcome of high-level talks in Vienna. Discussions of the Iranian nuclear talks often treat the P5+1 (the permanent five members of the U.N. Security Council plus Germany) side as a unified actor across the table from Tehran, but the P5+1 includes six great powers with diverging interests and perspectives on the negotiations. Maintaining the internal cohesion among these six players, as well as key allies such as Israel and Saudi Arabia, has been a central part of the long strategic bargaining process, one that will undoubtedly become even more important in the aftermath of a deal.

The P5+1 has maintained an impressive degree of unity thus far, but this accord may be tested as the talks reach a conclusion. It is not only Russian and Chinese views that color the talks. The European participants are also skeptical of U.S. intentions. During our recent research in several European capitals, it became clear that some there view the negotiations as a trading of interests between Iran and the United States, leaving them out of the process. However, the Europeans are crucial to the negotiations, the implementation of a potential deal and the viability and sustainability of the sanctions regime.

U.S. decision-makers should not take the European Union’s commitment and partnership to negotiations and the implementation of an agreement for granted or assume that if the talks are torpedoed in Washington, the E.U. will easily be on board to reimpose sanctions. If the U.S. Congress says it can act unilaterally and break down the process, it is ignoring the crucial foundations of the multilateral process. After all, Iran came back to the negotiating table in part due to sanctions, the strength and effectiveness of which stems from P5+1 unity. While many in Washington think the United States can and should act unilaterally, in reality, the United States needs and depends on the European Union for the implementation and sustainability of a final agreement on the Iranian nuclear program.

The divide should not be overly exaggerated. The European Union shares some of the same goals as the other parties to the negotiations. Much like the United States, the European Union wants to curb Iran’s nuclear program, ensuring that Tehran does not weaponize. But much like Iran, E.U. businesses in particular want sanctions lifted as soon as possible. This difference between the interests of E.U. politics and economics places Brussels in a difficult position. Nuances among the three lead European states involved in the talks — Germany, Britain and France — further complicate matters.

While Germany is the most eager to resolve the nuclear issue in order to reenter the Iranian market, Britain is cautious and more focused on nuclear restrictions than economic benefits of a deal. France is the most interesting case. While its government is notorious for its hard line in the negotiations, France’s businesses are biting at the bit to exploit the potential of the Iranian market. The French government’s position has been a source of contention not only between Paris and the rest of the P5+1 but also between the Foreign Ministry and other parts of government and businesses.

But recent U.S. activities have united the European Union in their discontent. In particular, many E.U. countries view Washington’s unilateral sanctions as advancing U.S. interests at the expense of E.U. businesses. A complaint often heard in E.U. capitals is that the United States dragged them to impose sanctions on Iran, creating real tensions among the 28 countries of the European Union. Recent unilateral efforts by the U.S. Congress to derail the talks have worsened these tensions. The European Union paid high costs for this process, and E.U. officials feel they are being undermined.

E.U. businesses in particular worry about the aftermath of an agreement. They fear that even if a deal lifts E.U. sanctions, U.S. sanctions may only be suspended. This means that the third-party component of U.S. sanctions remains. Businesses are naturally risk-averse and will not shoulder the costs of jeopardizing business with the United States to enter the Iranian market. European businesses have an edge, because they have more recent infrastructure and access in Iran that they can kick into gear. Their competitive position is stronger than assumed.

However, U.S. businesses are in a privileged position to navigate the post-sanctions minefield. They are used to working with the U.S. Treasury Department and its Office of Foreign Assets Control (OFAC) and better understand the complex web of U.S. legislation. If the Americans add to that advantage by pushing their own companies in, as the Europeans feel they are doing, the European Union would lag behind. E.U. businesses feel they will not be able to reap the benefits of reentering the Iranian market.

The United States is confident that regardless of its internal differences and their impact on the negotiations, the rest of the P5+1 will remain unified, but the aftermath of an agreement may change this. Should implementation of a potential agreement proceed flawlessly, there is no reason to challenge the P5+1. However, if there are hiccups along the way, the P5+1’s unity will be tested. Short of a glaring Iranian-caused implementation problem, Russia and China are unlikely to stay on board, and it would be difficult to convince them to reimpose sanctions on Iran.

But that is no surprise. E.U. commitment to reimpose sanctions will also be tested. While politically, the willpower might only be diminished rather than destroyed, economically it is a different matter. E.U. businesses will kick up a real fuss if they are ordered to take a step back, especially if that order comes from the United States. If the United States is the reason behind the implementation challenges in the first place — Congress taking unilateral action, for instance — E.U. partners may express even greater concern.

Serious attention needs to be paid to the demands of maintaining the coalition, both during these tense, final negotiations and in the implementation phase that follows. Preserving E.U. interests and the desire to work with the United States is crucial, especially as it will be increasingly difficult to get Russia and China on board should things go haywire. Sustained attention to these intra-alliance challenges will be essential for the success of any agreement.

For more analysis and background, see POMEPS Studies 13, Iran and the Nuclear Deal

Dina Esfandiary (@DEsfandiary) is a McArthur Fellow in the Centre for Science and Security Studies at King’s College London. Ariane Tabatabai (@ArianeTabatabai) is a visiting assistant professor in the Security Studies Program at the Georgetown University School of Foreign Service and a columnist for The Bulletin of the Atomic Scientists.