Few things are as explosive as the combination of power, ambition and anxiety — and there is plenty of all three in Riyadh these days.

Once a cautious and passive regional power, Saudi Arabia has found a new purpose in recent years. The ruthless ambition of Crown Prince Mohammed bin Salman, in full display at home with his crackdown on businessmen and members of the royal family, also radiates across the Middle East, driven by the urgency to check Iranian influence. Prince Mohammed has a point. Iran is set on becoming the dominant power from Iraq to Lebanon.

Saudi Arabia may exaggerate Iranian intentions and power, but Western and Asian countries typically understate them. The Iranians themselves are clear about how they view the region: “No decisive actions can be taken in Iraq, Syria, Lebanon, North Africa and the Gulf region without Iran’s consent,” Hassan Rouhani, Iran’s president, reportedly boasted last month. Tehran may not be in full control in Baghdad, Damascus and Beirut, but thanks to its proxies and allies, it can decisively shape their battlefields and politics.

Given these circumstances, Prince Mohammed has good reason to question the value of his predecessors’ risk aversion on foreign policy. Under previous kings, Riyadh was indeed keen to reach out to Tehran despite provocative Iranian actions, including fast-tracking its nuclear program just as King Abdullah courted Presidents Akbar Rafsanjani and Mohammad Khatami, and plotting to assassinate a Saudi ambassador in the United States.

Now Saudi foreign and security policy has gone into overdrive. Rather than carefully pushing back Iran and enrolling broad support for this effort, the approach has been haphazard, unsettling and counterproductive — and Iran remains one step ahead.

Saudi Arabia’s intervention in Yemen, on behalf of the government forces fighting against Iranian-backed Houthi rebels has been costly and inconclusive, even after more than two and a half years. In fact, it could lead to the very outcome that Riyadh most wanted to prevent: the transformation of the Houthi movement into something akin to Lebanon’s Hezbollah — except much closer to Saudi borders. Indeed, unless the war in Yemen comes to an end soon, those well-armed, Iran-backed militants will soon sit atop a shattered state and a starved society.

The Saudi-led blockade of Qatar has been more successful. The effort to tame that country’s assertive regional policies has worked and the crisis has now been put on the back burner of international diplomacy. That said, the reputational cost has been high for all: A dispute framed by the Saudis as a struggle for the future of the Middle East is seen in many capitals as an unnecessary and disruptive clash of wealthy royals.

The latest Saudi venture — the forced resignation as prime minister of Lebanon and probable house arrest of Saad Hariri, once a favorite ally of Riyadh — has bewildered many in Lebanon and elsewhere. It is also likely to backfire. This move plays into the hands of Iran and Hezbollah, who duplicitously pose as rule abiding, despite having undermined the Lebanese state for decades, assassinating rivals, plunging the country into foreign wars and exporting fighters across the region. In contrast, Saudi Arabia was backing state institutions and working through established politicians like Mr. Hariri. What Riyadh has now in mind — and in store — for Lebanon is unclear.

In fact, if its goal is to counter Iran, Riyadh is picking the wrong battlefields.

Lebanon and Yemen are peripheral countries, where wars are costly and complex, outcomes ambiguous and returns low. In the Middle East, the balance of power is determined in Syria and Iraq. But in those countries, the costs are high and the risks even higher. And in both places, Iran is well ahead.

In 2011, Riyadh reasoned that sponsoring the Syrian rebellion would help compensate for Iran’s dominance in Iraq. It hasn’t worked that way. The faltering Syrian insurgency cannot be revived, the United States has basically washed its hands of the Syrian civil war, and the country’s future is being decided in Moscow, Ankara and Tehran.

Perhaps better news for the Saudis is in Iraq, where they are making a late comeback after denying the new political realities since 2003. The courting of Moktada al-Sadr, a firebrand cleric turned a populist critic of Iran’s role in Iraq, and the embrace of Haider al-Abadi, the Iraqi prime minister, are spectacular yet tentative moves to balance Tehran’s influence.

Riyadh has learned the hard way that regional alliances, cultivated at great cost, don’t necessarily deliver the expected political and military benefits. The Saudis have propped up the government of President Abdel Fattah el-Sisi in Egypt with billions of dollars, yet Mr. Sisi is now resuming relations with President Bashar al-Assad of Syria and rejecting Saudi pressure to escalate tensions with Iran. Mr. Sisi has also declined Saudi requests to send troops to Yemen to fight against the Houthis.

A notable success for Saudi Arabia has been the realignment of American and Saudi policies, which is why President Trump has been welcomed so theatrically in the kingdom. Riyadh has been traumatized by its experience with the Obama administration, in particular President Obama’s unreciprocated eagerness to repair relations with Tehran and teach Iran and Saudi Arabia to “share” the Middle East.

Riyadh and Washington undeniably converge right now when it comes to Iran but that does not amount to a common strategy. Indeed, President Trump, Jared Kushner and Prince Mohammed seem to mistake presidential and princely preference and mutual agreement for statecraft and implementation.

Fundamentally, who prevails in the rivalry between Tehran and Riyadh comes down to capacity and competence. Iran has the networks, expertise, experience and strategic patience required to fight and win proxy wars at low cost and with plenty of disingenuous deniability. The Saudis simply don’t, which is why seeking to beat the Iranians at this game is dangerous and costly.

Iran has another strength: It has demonstrated that it will be there for its friends and allies in good and bad times. Saudi Arabia does not have the same constancy. Just ask Syrian rebels, Iraqi tribal leaders and Lebanese politicians.

Being mostly right on the Iranian menace is not enough. Rolling back Iran, a worthy and urgent goal, will require a broad international consensus and a less aggressive Saudi Arabia. Furthermore, unattainable foreign ambitions distract, at great cost, from the more important and momentous task of internal reform.