Iranian supreme leader Ayatollah Ali Khamenei speaks to Iranian government officials in Tehran on June 23, 2015. ( /Supreme leader’s official Web site via European Pressphoto Agency)

Despite threats, isolation and sanctions, Iran over the past 15 years has developed the capability to enrich uranium that could be used for nuclear weapons.

The question now is whether Iran can be prevented from taking the next steps and producing weapons-grade uranium and the accompanying hardware to build nuclear bombs or missile warheads.

If the United States and its negotiating partners reach an agreement with Iran this week regarding Tehran’s nuclear program, proponents will praise what emerges and opponents will pick it apart, section by section.

Each side will present arguments, but it will be the facts that are important, and they will be difficult for the public to determine.

Take the simple question of why two years ago Iran suddenly seemed willing to talk about reaching an agreement on its nuclear program.

“It was actually both the fact of sanctions in 2013 and the threat of even tighter sanctions that drove them to the negotiating table,” Sen. Tom Cotton (R-Ark.) said Sunday on ABC’s “This Week.” Cotton, a critic of the Obama administration’s approach to the negotiations, added: “That’s why we shouldn’t have let up those sanctions. We should have insisted on the very simple terms that President Obama himself proposed at the outset of this process — Iran dismantles its nuclear program entirely, and then they will get sanctions relief.”

Iran’s leadership said it moved to negotiate for a much different reason — it had built the nuclear capability it wanted.

“The day that we entered these negotiations we had an acceptable and important achievement. We felt that we are entering [the negotiations] with our hands full,” Iran’s supreme leader, Ayatollah Ali Khamenei, said in a speech to senior government officials just two weeks ago.

Noting that the U.S. and other nuclear powers after the 1979 overthrow of the shah had refused to provide revolutionary Iran with 20 percent enriched nuclear fuel for the Tehran research reactor, Khamenei said Iranian scientists took “the other side by surprise; they produced 20 percent [fuel].”

He then pointed out “that in nuclear enrichment, the important and difficult part is to move from 3 percent and 4 percent [enrichment] to 20 percent. From 20 percent to 90 percent [enrichment] is a very simple move.”

U.S. intelligence agreed. On Jan. 31, 2012, Director of National Intelligence James R. Clapper told the Senate Intelligence Committee in a public hearing: “Iran has the scientific, technical and industrial capacity to eventually produce nuclear weapons, making the central issue its political will to do so. These [technical] advancements contribute to our judgment that Iran is technically capable of producing enough highly enriched uranium for a weapon, if it so chooses.”

Sanctions did have an effect. Khamenei said they led to policies called “the economy of resistance,” which made Iranians look inward, encouraged increased “saving . . . consumption of domestic products.”

Khamenei described the negotiations as “a give-and-take,” adding: “We have no problem in this regard to give something [and in return] take something in order for sanctions to be removed. However, the nuclear industry should not be stopped, should not be damaged.”

Put that up against Cotton’s view Sunday of the goal of negotiations. “Iran should have faced a simple choice: They dismantle their nuclear program entirely, or they face economic devastation and military destruction of their nuclear facilities.”

The Obama administration, members of Congress, presidential contenders, think tanks and experts will all be commenting on any agreement. Think back to April when the State Department produced a four-page fact sheet on the framework agreement’s contents and the Iranian Foreign Ministry responded with one of its own — competing interpretations of what had been agreed to.

Most attention will now be focused on timing for withdrawal of various sanctions and implementation of inspections along with mechanisms to determine violations and responses to noncompliance.

Khamenei in April was quoted as saying all sanctions had to be removed when the deal was signed. Two weeks ago he called for “economic and financial and banking sanctions” to be withdrawn on signing but said later: “Of course, removal of sanctions has certain executive stages — we accept this. Executive aspects of removal of sanctions should be proportionate to the executive [steps] that Iran will undertake — part of this in return for part of that, another part of this in return for another part of that.”

The same ambiguity exists in Iranian statements about the inspection and verification fields. Let’s see what the agreement, if reached, calls for.

From the U.S. standpoint, two steps appear necessary. Congress should set up a joint House-Senate oversight panel to focus on implementation of any agreement, thereby heading off at least four or more committees in each body taking on overlapping inquiries.

The White House needs some interagency group, probably centered in the National Security Council, to handle issues as they arise in Congress — which already has voted for six-month reports — and from the public. There is an obvious need for a public spokesperson and public affairs group just to deal with the agreement.

One other item will come up: The International Atomic Energy Agency (IAEA) will need additional funds and resources to handle the inspection and analysis effort required by any new agreement. IAEA Director General Yukiya Amano has already suggested he would have to double the number of inspectors in Iran based on his understanding of what his group would need to do.

If a deal is struck, it will be only the beginning of a complex diplomatic and political process and not the end.

For previous Fine Print columns, go to washingtonpost.com/fedpage.