US authorities frequently discuss "deterring" Iran. In actuality, what does that mean?

Does the US still possess the ability to dissuade its enemies? 

President Joe Biden stated that although the US would continue to respond to attacks linked to Iran "at times and places of our choosing," it "does not seek conflict in the Middle East or anywhere else" when the US launched airstrikes earlier this month against the proxy militias linked to Iran's Revolutionary Guard Corps (IRGC) in retaliation for the attack that killed three US soldiers at a base in Jordan. Almost exact words were used in Secretary of Defense Lloyd Austin's announcement regarding the strikes.
It is reasonable for readers of these words to wonder how bombing 85 targets and killing almost 40 people can be done without "seeking conflict." Not exactly tools of Gandhian nonviolent resistance, these B-1 bombers. The official reason for the strikes was to "deter the IRGC and affiliated militia groups from conducting or supporting further attacks on United States personnel and facilities," as stated in the letter the White House issued to Congress in compliance with the War Powers Resolution. 

The overarching principle of US strategy since the terrorist attacks on October 7, 2001, and the war in Gaza that followed, has been "deterrence"—more especially, preventing Iran and its numerous proxies from starting a larger regional conflict.Pentagon sources stated that the goal of sending more US military equipment, including aircraft carriers, to the Middle East last fall was to "deter a broader conflict." The US began military operations in Yemen in January with the intention of “deterring Houthi attacks in the Red Sea.” 

Language like this is not brand-new. President Donald Trump cited creating a "strong deterrent" against the use of chemical weapons as his primary justification for carrying out airstrikes against Bashar al-Assad's government in Syria in 2018. In spite of his efforts to come to a nuclear agreement with Iran, President Barack Obama gave apprehensive American friends in the Persian Gulf the assurance in 2016 that he would not think twice about using military force to "deter" Iranian aggression.
The results of all this deterrent action have been decidedly mixed, at least in recent examples. Iran has not yet used its own forces to launch a direct attack on US military installations in Israel. Iran's most potent proxy, Hezbollah, which is located in Lebanon, has not yet attacked Israel on a large scale, contrary to what many had feared earlier in the conflict. However, during this time, the Houthis, who are backed by Iran, have continued their attacks on shipping, claiming that Western airstrikes “will not deter us.” Additionally, Hezbollah and Israel have been exchanging fire more frequently, though it is still far from reaching full-scale conflict. Finally, US troops in the region have been the target of numerous attacks that have resulted in those three deaths and numerous injuries.
US government spokespeople are now frequently asked if US deterrence in the Middle East has failed at news conferences. It's a topic with implications that go beyond this region, too, as more and more longstanding allies begin to doubt the worth of US security promises as conflicts across the globe increase. Is the United States still able to intimidate its enemies in the modern world?
Carrying a big stick
Since at least the time of the ancient Greeks, the idea of deterrence—that is, preventing an enemy from acting by the prospect of punishment—has been a part of military strategy and international relations. Theorists like Nobel Prize-winning economist Thomas Schelling expanded on the idea during the height of the Cold War with his 1966 book Arms and Influence, which explored how nations can use military might to influence one another's actions. In the nuclear age, this might include the ability to kill millions of people almost instantly. 

In the nuclear age, Schelling depicted superpower rivalry as a “competition in risk taking, characterized not so much by tests of force as by tests of nerve.”
The conflict that is now raging in Ukraine serves as a master class in risk management. Russia has had to put up with Western military assistance worth billions of dollars streaming into Ukraine during the crisis, greatly enhancing that nation's military might. Crucially, nations who are part of NATO, like as Poland, are providing this aid. These nations are bound by a treaty that states that any attack on one member of the alliance constitutes an attack on all of them. It is clear that Vladimir Putin believes that assurance to be genuine and does not want to strike the relief shipments and start a shooting conflict with the entire alliance. 

Deterrence, however, works both ways. The member nations of NATO also wish to prevent a direct conflict with Russia, especially in view of Putin's repeated threats to employ nuclear weapons.
Putin's threats have lost credibility, though, as the West has steadily increased the amount of aid it has given to Ukraine over time without Russia launching a disastrous nuclear counterattack. Thus, Ukraine is currently getting aid that would have been unimaginable in the early stages of the conflict, when, by all accounts, US security officials gave greater importance to keeping the battle contained inside Ukraine than to Ukrainian triumph. This aid includes tanks, fighter jets, and targeting support. Schelling and others have called this kind of escalation "salami tactics," which means chopping off one small slice at a time, degrading your opponent's red line. It is incremental, but never spectacular enough to force your opponent to react.
The limits of deterrence
It is neither true or accurate to state that the increased US military presence in the Middle East has not been able to deter Iran, according to Bilal Saab, a former Pentagon officer who is currently employed with the Middle East Institute. He told Vox, "You probably would have seen a whole lot more activity from Iran to spread its influence without being checked if there weren't that forward deployed immediate deterrence in the region." Saab listed several far more provocative measures that Iran could have taken, such as closing the Strait of Hormuz, which would have had a greater effect on world energy supplies than the disruptions the Houthis have caused in their attacks on Red Square, or an attack that killed many more US soldiers than just a few.
"Proving that deterrence is working is difficult because, at the end of the day, you can't prove something that didn't happen," Saab admitted. 

However, the Foundation for the Defense of Democracies reports that Iranian proxy groups have launched over 160 strikes against US military targets in the Middle East since October, demonstrating that Tehran has undoubtedly not been stopped from carrying out some activities. 

Senior fellow at the Stimson Center Emma Ashford said, "It's basically become the status quo: a proxy group lobs rockets at US bases, then US forces bomb and kill the proxies."
The future of deterrence
US deterrence has recently been under fire in a number of regions, not only the Middle East. The former president and current presidential contender, Donald Trump, declared last week at a rally in South Carolina that he would urge Russia to "do whatever the hell they want" to any NATO member that has not met its defense budget obligations. While Trump is not the only one demanding that certain European nations accomplish those goals, NATO is not a country club where membership is exclusive to those who can afford dues. Trump's remarks seriously questioned the mutual defense clause, which has up until now been effective in discouraging Russia from striking NATO members directly.
"Extended deterrence" refers to the ability to employ the threat of military action to keep rivals and allies from invading not just your own territory but also their land. However, this strategy only works if both parties are confident that you would honor your agreements. That credibility is hard to sustain in the hyperpartisan US of today, when there's little agreement on fundamental national security priorities or continuity between administrations, as demonstrated by Congress's present failure to enact aid packages for Taiwan, Israel, and Ukraine.

"Washington can no longer be treated as a somewhat predictable entity by any country that is in a treaty relationship or treaty alliance with the United States," stated Ankit Panda, a senior scholar in the Carnegie Endowment for International Peace's nuclear policy program. Allies are aware that Biden will protect "every inch" of NATO territory, just as President Obama and all other postwar presidents have. However, they increasingly understand that the US's future course will be determined by the party in charge of Congress and the White House come 2025 and beyond.