A tectonic shift has occurred in the U.S.-Saudi relationship. Despite significant pressure from the Obama administration to remain on the sidelines, Saudi leaders sent troops into Manama in March to defend Bahrain’s monarchy and quell the unrest that has shaken that country since February. For more than 60 years, Saudi Arabia has been bound by an unwritten bargain: oil for security. Riyadh has often protested but ultimately acquiesced to what it saw as misguided U.S. policies. But American missteps in the region since Sept. 11, an ill-conceived response to the Arab protest movements and an unconscionable refusal to hold Israel accountable for its illegal settlement building have brought this arrangement to an end. As the Saudis recalibrate the partnership, Riyadh intends to pursue a much more assertive foreign policy, at times conflicting with American interests.

The backdrop for this change are the rise of Iranian meddling in the region and the counterproductive policies that the United States has pursued here since Sept. 11. The most significant blunder may have been the invasion of Iraq, which resulted in enormous loss of life and provided Iran an opening to expand its sphere of influence. For years, Iran’s leadership has aimed to foment discord while furthering its geopolitical ambitions. Tehran has long funded Hamas and Hezbollah; recently, its scope of attempted interference has broadened to include the affairs of Arab states from Yemen to Morocco. This month the chief of staff of Iran’s armed forces, Gen. Hasan Firouzabadi, harshly criticized Riyadh over its intervention in Bahrain, claiming this act would spark massive domestic uprisings.

Such remarks are based more on wishful thinking than fact, but Iran’s efforts to destabilize its neighbors are tireless. As Riyadh fights a cold war with Tehran, Washington has shown itself in recent months to be an unwilling and unreliable partner against this threat. The emerging political reality is a Saudi-led Arab world facing off against the aggression of Iran and its non-state proxies.

Saudi Arabia will not allow the political unrest in the region to destabilize the Arab monarchies — the Gulf states, Jordan and Morocco. In Yemen, the Saudis are insisting on an orderly transition of power and a dignified exit for President Ali Abdullah Saleh (a courtesy that was not extended to Hosni Mubarak, despite the former Egyptian president’s many years as a strong U.S. ally). To facilitate this handover, Riyadh is leading a diplomatic effort under the auspices of the six-country Gulf Cooperation Council. In Iraq, the Saudi government will continue to pursue a hard-line stance against the Maliki government, which it regards as little more than an Iranian puppet. In Lebanon, Saudi Arabia will act to check the growth of Hezbollah and to ensure that this Iranian proxy does not dominate the country’s political life. Regarding the widespread upheaval in Syria, the Saudis will work to ensure that any potential transition to a post-Assad era is as peaceful and as free of Iranian meddling as possible.

Regarding Israel, Riyadh is adamant that a just settlement, based on King Abdullah’s proposed peace plan, be implemented. This includes a Palestinian state with its capital in East Jerusalem. The United States has lost all credibility on this issue; after casting the sole vote in the U.N. Security Council against censuring Israel for its illegal settlement building, it can no longer act as an objective mediator. This act was a watershed in U.S.-Saudi relations, guaranteeing that Saudi leaders will not push for further compromise from the Palestinians, despite American pressure.

Saudi Arabia remains strong and stable, lending muscle to its invigorated foreign policy. Spiritually, the kingdom plays a unique role for the world’s 1.2 billion Muslims — more than 1 billion of whom are Sunni — as the birthplace of Islam and home of the two holiest cities. Politically, its leaders enjoy broad domestic support, and a growing nationalism has knitted the historically tribal country more closely together. This is largely why widespread protests, much anticipated by Western media in March, never materialized. As the world’s sole energy superpower and the de facto central banker of the global energy markets, Riyadh is the economic powerhouse of the Middle East, representing 25 percent of the combined gross domestic product of the Arab world. The kingdom has amassed more than $550 billion in foreign reserves and is spending more than $150 billion to improve infrastructure, public education, social services and health care.

To counter the threats posed by Iran and transnational terrorist networks, the Saudi leadership is authorizing more than $100 billion of additional military spending to modernize ground forces, upgrade naval capabilities and more. The kingdom is doubling its number of high-quality combat aircraft and adding 60,000 security personnel to the Interior Ministry forces. Plans are underway to create a “Special Forces Command,” based on the U.S. model, to unify the kingdom’s various special forces if needed for rapid deployment abroad.

Saudi Arabia has the will and the means to meet its expanded global responsibilities. In some issues, such as counterterrorism and efforts to fight money laundering, the Saudis will continue to be a strong U.S. partner. In areas in which Saudi national security or strategic interests are at stake, the kingdom will pursue its own agenda. With Iran working tirelessly to dominate the region, the Muslim Brotherhood rising in Egypt and unrest on nearly every border, there is simply too much at stake for the kingdom to rely on a security policy written in Washington, which has backfired more often than not and spread instability. The special relationship may never be the same, but from this transformation a more stable and secure Middle East can be born.

A day after Saudi Arabia’s military rolled into Bahrain, the Iranian government branded the move “unacceptable” on Tuesday, threatening to escalate a local political conflict into a regional showdown with Iran.

“The presence of foreign forces and interference in Bahrain’s internal affairs is unacceptable and will further complicate the issue,” Ramin Mehmanparast, the Iranian Foreign Ministry spokesman told a news conference in Tehran, according to state-run media.

Even as predominantly Shiite Muslim Iran pursues a determined crackdown against dissent at home, Tehran has supported the protests led by the Shiite majority in Bahrain.

“People have some legitimate demands and they are expressing them peacefully,” Mr. Memanparast said. “It should not be responded to violently.”

“We expect their demands be fulfilled through correct means,” Mr. Mehmanparast added. Iran’s response — while anticipated — showed the depth of rivalry across the Persian Gulf in a contest that has far-reaching consequences in many parts of the Middle East.

On Monday, Iranian state-run media went so far as to call the troop movement an invasion. Saudi Arabia has been watching uneasily as Bahrain’s Shiite majority has staged weeks of protests against a Sunni monarchy, fearing that if the protesters prevailed, Iran, Saudi Arabia’s bitter regional rival, could expand its influence and inspire unrest elsewhere.

The Saudi decision to send in troops on Monday could further inflame the conflict and transform this teardrop of a nation in the Persian Gulf into the Middle East’s next proxy battlefield between regional and global powers. On Tuesday, there was no immediate indication that the Saudi forces were confronting protesters in the central Pearl Square — the emblem of the Bahrain protest much as Cairo’s Tahrir Square assumed symbolic significance in the Egyptian uprising.

Several hundred protesters camped out there on what seemed initially to be a quiet day with little traffic on the streets as the details of the deployment by Bahrain’s neighbors — and their mission — remained ill-defined.

On Monday, about 2,000 troops — 1,200 from Saudi Arabia and 800 from the United Arab Emirates — entered Bahrain as part of a force operating under the aegis of the Gulf Cooperation Council, a six-nation regional coalition of Sunni rulers that has grown increasingly anxious over the sustained challenge to Bahrain’s king, Hamad bin Isa al-Khalifa. “This is the initial phase,” a Saudi official said. “Bahrain will get whatever assistance it needs. It’s open-ended.”

The decision is the first time the council has used collective military action to help suppress a popular revolt — in this case a Shiite popular revolt. It was rejected by the opposition, and by Iran, as an “occupation.” Iran has long claimed that Bahrain is historically part of Iran.

The troops entered Bahrain at an especially combustible moment in the standoff between protesters and the monarchy. In recent days protesters have begun to move from the encampment in Pearl Square, the symbolic center of the nation, to the actual seat of power and influence, the Royal Court and the financial district. As the troops moved in, protesters controlled the main highway and said they were determined not to leave.

“We don’t know what is going to happen,” Jassim Hussein Ali, a member of the opposition Wefaq party and a former member of Parliament, said in a phone interview. “Bahrain is heading toward major problems, anarchy. This is an occupation, and this is not welcome.”

Rasool Nafisi, an academic and Iran expert based in Virginia, said: “Now that the Saudis have gone in, they may spur a similar reaction from Iran, and Bahrain becomes a battleground between Saudi and Iran. This may prolong the conflict rather than put an end to it, and make it an international event rather than a local uprising.”

An adviser to the United States government, who spoke on the condition of anonymity because he was not authorized to talk to the press, agreed. “Iran’s preference was not to get engaged because the flow of events was in their direction,” he said. “If the Saudi intervention changes the calculus, they will be more aggressive.”

Though Bahrain said it had invited the force, the Saudi presence highlights the degree to which the kingdom has become concerned over Iran’s growing regional influence, and demonstrates that the Saudi monarchy has drawn the line at its back door. Oil-rich Saudi Arabia, a close ally of Washington, has traditionally preferred to operate in the shadows through checkbook diplomacy. It has long provided an economic lifeline to Bahrain.

But it now finds itself largely standing alone to face Iran since its most important ally in that fight, Hosni Mubarak of Egypt, has been ousted in a popular uprising. Iran’s ally, Hezbollah, recently toppled the Saudi-backed government of Lebanon — a symbol of its regional might and Saudi Arabia’s diminishing clout.

But Bahrain is right at Saudi Arabia’s eastern border, where the kingdoms are connected by a causeway.

The Gulf Cooperation Council was clearly alarmed at the prospect of a Shiite political victory in Bahrain, fearing that it would inspire restive Shiite populations in Saudi Arabia and Kuwait to protest as well. The majority of the population in Saudi Arabia’s eastern provinces, where the oil is found, is Shiite, and there have already been small protests there.

“If the opposition in Bahrain wins, then Saudi loses,” said Mustafa el-Labbad, director of Al Sharq Center for Regional and Strategic Studies in Cairo. “In this regional context, the decision to move troops into Bahrain is not to help the monarchy of Bahrain, but to help Saudi Arabia itself .”

The Bahrain government said that it had invited the force in to help restore and preserve public order. The United States — which has continued to back the monarchy — said Monday that the move was not an occupation. The United States has long been allied with Bahrain’s royal family and has based the Navy’s Fifth Fleet in Bahrain for many years.

Though the United States eventually sided with the demonstrators in Egypt, in Bahrain it has instead supported the leadership while calling for restraint and democratic change. The Saudi official said the United States was informed Sunday that the Saudi troops would enter Bahrain on Monday.

Saudi and council officials said the military forces would not engage with the demonstrators, but would protect infrastructure, government offices and industries, even though the protests had largely been peaceful. The mobilization would allow Bahrain to free up its own police and military forces to deal with the demonstrators, the officials said.

The Gulf Cooperation Council “forces are not there to kill people,” said a Saudi official, speaking on the condition of anonymity because he was not authorized to talk to the press. “This is a G.C.C. decision; we do not violate international law.”

But the officials also acknowledged that it was a message to Iran. “There is no doubt Iran is involved,” said the official, though no proof has been offered that Iran has had anything to do with the political unrest.

Political analysts said that it was likely that the United States did not object to the deployment in part because it, too, saw a weakened monarchy as a net benefit to Iran at a time when the United States wants to move troops out of Iraq, where Iran has already established an influence.

The military force is one part of a Gulf Cooperation Council effort to try to contain the crisis in Bahrain that broke out Feb. 14, when young people called for a Day of Rage, fashioned after events in Egypt and Tunisia. The police and then the army killed seven demonstrators, leading Washington to press Bahrain to remove its forces from the street.

The royal family allowed thousands of demonstrators to camp at Pearl Square. It freed some political prisoners, allowed an exiled opposition leader to return and reshuffled the cabinet. And it called for a national dialogue.

But the concessions — after the killings — seemed to embolden a movement that went from calling for a true constitutional monarchy to demanding the downfall of the monarchy. The monarchy has said it will consider instituting a fairly elected Parliament, but it insisted that the first step would be opening a national dialogue — a position the opposition has rejected, though it was unclear whether the protesters were speaking with one voice.

The council moved troops in after deciding earlier to help prop up the king with a contribution of $10 billion over 10 years, and said that it might increase that figure. But if the goal was to intimidate Iran, or the protesters, that clearly was not the first response.

Bahrain’s opposition groups issued a statement: “We consider the entry of any soldier or military machinery into the Kingdom of Bahrain’s air, sea or land territories a blatant occupation.”