It’s clear that nobody in the Congress really trusts Iran.

But whether lawmakers approve the agreement to stem the Islamic republic’s nuclear ambitions may instead come down to whether they trust the United States’s oldest ally: Europe.

Sen. Robert Menendez (D-N.J.) (Reuters/Eduardo Munoz)

“I’d like to be able to trust Europe, but when they are frothing at the mouth about business interests, especially once those businesses interests are really entrenched, will it be so easy to call a violation a violation?” Sen. Robert Menendez (D-N.J.) asked. “That’s the question.”

For the Iran deal to fully work, the United States has to cede a good amount of control to other countries and international organizations.

For example, Iran isn’t allowing any Americans on the International Atomic Energy Agency (IAEA) teams that will inspect nuclear sites. The United Nations Security Council approved the deal before Congress weighed in, to the chagrin of lawmakers on both sides of the aisle. And the United States has just one voice out of eight on the commission created to help resolve disputes about compliance and broker logjams over requests for the IAEA to inspect additional sites — processes that could eventually give rise to a “snap back” of international sanctions.

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But where Americans are being kept at arm’s length, Europeans will be allowed in.

European IAEA inspectors will be able to examine suspicious sites. And Europe holds four of the seats on that eight-member joint commission — enough for the United Kingdom, France, Germany and the European Union to carry any U.S.-led majority against Russia, China and Iran in disputes over compliance, provided the Western nations hold rank.

The thing is, not all lawmakers are so sure they will.

As lawmakers pore over the details of the Iran deal to determine whether U.S. security interests are protected, they are worrying about whether the U.S. has enough like-minded friends to ensure that it can keep the deal, if implemented, on track for the long haul.

Republicans especially are not comfortable trusting other countries — even allies — to look out for U.S. interests. One reason is that eliminating sanctions on Iran has some obvious economic benefits for a Europe seeking new market opportunities and alternatives to Russian oil.

“The concern that I have is that when you deal with some of these issues, there’s a lot of economic benefits that happen in Europe that don’t happen here,” said Sen. James Lankford (R-Okla.). “We don’t really have a strong voice.”

Other Republicans simply believe that since the end of the Cold War, the United States and Europe are suffering a deepening divide in their philosophy toward adversaries.

“Europe is behaving in the finest traditions of Neville Chamberlain, and Ukraine is the classic example,” said Sen. John McCain (R-Ariz.), complaining that the U.S. was no longer “respected and influential” enough in the world to lead its European allies.

“With a diminished role in the world and a lack of leadership, the Europeans may attempt to go their own way on Iran. But if there had been a President Reagan, there would have been a penalty for that.”

Europe’s preferences have already influenced U.S. dealmaking, pushing negotiators to go further than they would have probably gone on their own. The eventual lifting of the 2010 international embargo on conventional arms was not something the United States wanted to cede. But it did, following mounting pressure not just from Russia and China, but from some European countries as well.

Many lawmakers believe that if the U.S. puts its foot down and refuses to uphold the deal, Europe will not endorse a continuation of sanctions as a show of loyalty to an old ally.

“What are we left with if we walk away from this deal? I’m not sure that we will hold the European countries to sanctions,” said Sen. Claire McCaskill (D-Mo.)

But for most Democrats, not seeing eye to eye with Europe about sanctions doesn’t mean a lack of trust when it comes to the rest of the deal.

Europe is “not being coy about whether or not they want to stay part of a rigid sanctions regime. They have said and done things that show they’re not willing to do that. But I would take our European allies at their word,” McCaskill continued. “They’re as adamant as we are about Iran not getting nuclear weapons. So it’s in their best interest to help us with this aggressive verification process.”

Most Democrats argued that Europe won’t let their business interests dictate whether Iran is complying with the terms of the deal.

“It’s hard for me to imagine a scenario in which we’re going to have a major break with our European allies on a question of inspections,” said Sen. Chris Murphy (D-Conn.), a member of the Senate Foreign Relations committee.

“The Europeans and the United States will support the IAEA — we always have,” said Sen. Ben Cardin (D-Md.), ranking member on the Senate Foreign Relations committee. “So that seems like it’s a fair process.”

Neither the Iran deal nor the U.S. relationship with Europe exists in a vacuum, of course. Even lawmakers cheering the deal check their enthusiasm at Iran’s other political activities and worry that a richer Iran might be bolder about funding groups terrorizing U.S. allies, especially in the Middle East. Likewise, the United States and Europe’s relationship is sure to weather a few tests in other geopolitical arenas, especially as the continental powers determine whether they can continue a coordinated diplomatic and sanctions policy against Russia.

Those other issues are sure to influence lawmakers as they begin to examine the Iran deal over the next 60 days.

“Britain, France, Germany, sure. But the EU?” said Sen. Chris Coons (D-Del.), ticking off the potential U.S. allies under the deal, and pausing to note that he had no idea who in the politically diverse EU would be involved in its decisionmaking. “I don’t know enough about that, and I should. Because we’re counting all of those as automatic yesses, and they’re not, necessarily.”

Ultimately, a decision to trust another country involves a choice to accept a degree of uncertainty. But one lawmaker suggested that putting faith in potentially untrustworthy allies might actually be a good exercise.

“So we don’t trust them, and they don’t trust us. If there’s a lack of trust, let’s not worry about the trust angle, let’s just put something together that can basically work and be verified,” said Sen. Joe Manchin (D-W.Va.). “If this coalition can stick together for the betterment of the world, then this can be very productive, very favorable — unless you’re just looking for another war, which I’m not.”