Michael Singh is managing director of the Washington Institute for Near East Policy. From 2005 to 2008, he worked on Middle East issues at the National Security Council.

Recently, red lines both figurative and — since Israeli Prime Minister Benjamin Netanyahu’s prop-assisted speech to the United Nations two weeks ago — literal have come to dominate the Iran policy discussion. While Netanyahu was as explicit as possible in his delineation of Israel’s red line regarding Iran’s nuclear status, President Obama has been reluctant to draw one. Instead, he has offered different, even contradictory, messages to two audiences: To Israel, he has pleaded for patience; to Iran and reluctant U.S. allies, he has warned that “time is not unlimited.”

Problem is, neither audience seems to believe him. To address this, he should communicate clearer limits on American forbearance by setting his own red lines for Iran.

While red lines have been mischaracterized as automatic triggers or even deadlines for war, their purpose is to facilitate diplomacy. Generally speaking, such lines set bounds for action by indicating what Washington will and won’t tolerate. Red lines set by the United States are crucial for determining the “rules of the game” in geopolitics — not the formal rules set by multilateral bodies such as the United Nations, but the informal rules that just as clearly guide action by states. Red lines create predictability and can also foster stability by heading off avoidable conflicts and forming the context for diplomacy.

To work, red lines must possess two characteristics: enforceability and credibility. Enforceability means the line must correspond to an action that can be detected and then countered or prevented; credibility means that others believe we will enforce the line once it is trespassed.

The U.S. red line on Iran — that Iran simply cannot have a nuclear weapon — falls short on both counts. It is not enforceable because once Tehran gets sufficiently close to possessing a nuclear weapon, such as by producing a stockpile of weapons-grade enriched uranium, the final steps required to make a nuclear weapon can probably be done relatively quickly and in secret — and thus are not detectable.

The U.S. red line also, regrettably, lacks credibility. Washington did not move to halt the North Korean or Syrian nuclear programs; we did so in Iraq but at so high a price that “avoiding another Iraq” has practically become a mantra of U.S. foreign policy. Other states are understandably skeptical that we would move to stop Iran from acquiring nuclear weapons if and when the moment finally comes.

For its part, Iran is following a canny strategy that serves to further undermine U.S. red lines. On the one hand, Tehran is moving incrementally toward nuclear capability, avoiding dramatic steps that could provoke an international outcry and lead the United States to believe that our red line had been crossed. Instead, Tehran is careful to acclimate us to a new “nuclear normal” at each step of its progress, moving forward only when its accomplishments have been accepted as a fait accompli.

Furthermore, Iran warns unstintingly of the negative consequences of a military conflict, warnings that are accepted and repeated by credulous Western analysts and officials. This makes it seem a remote possibility that those officials would ever order a strike on Iran.

In his U.N. speech, Netanyahu articulated a clear red line: Iran accumulating one bomb’s worth of “medium-enriched” uranium. Whether that is the right line depends on two things. First, does it satisfy the criteria required of any red line — credibility and enforceability? There can be little doubt of Israel’s credibility, given its history of preemptive strikes on adversaries’ nuclear programs. But enforceability is another matter; while the line is detectable, many — including Iran — doubt Israel’s capacity to counter it alone.

Because multiple red lines meet these criteria, a second question must be asked: Does Netanyahu’s red line best satisfy our twofold policy objective of preventing Iran from acquiring a nuclear weapon while delaying a military conflict as long as possible?

Given that Obama administration officials have publicly expressed doubts about Israel’s capacity to destroy Iran’s nuclear infrastructure and have argued that the United States would have sufficient time to act militarily even after Iran has progressed beyond the point Netanyahu identified, it is unlikely that they find his red line satisfactory.

After Netanyahu’s clear statement, however, it is up to the Obama administration to suggest a red line that better meets U.S. objectives as well as the criteria of enforceability and credibility. And when it comes to credibility, the United States has undermined itself on multiple fronts — by rewarding Iranian defiance with better offers at the negotiating table, by enforcing sanctions reluctantly and by allowing senior officials to speak out publicly against the military option that the president insists remains “on the table.”

Obama is right to want to avoid conflict with Iran. But lest his patience be mistaken for apprehension, and allies and adversaries alike tune out U.S. warnings, he would be wise to overcome his aversion to red lines.