KARACHI, Pakistan—The waters around the Arabian Peninsula have calmed for the moment, but preparations for combat continue, with joint exercises and security conferences showing just how profoundly the region’s strategic balances are shifting.

As confidence declines in U.S. President Donald J. Trump’s ability to navigate the difficult moral and military choices in the region, new players are entering the picture in and around the Persian Gulf.

An Israeli delegation attended a U.S.-backed maritime-security conference that began Sunday in Manama with delegations from Saudi Arabia and Bahrain as a vast international maritime exercise, IMX 19, got under way in the Persian Gulf. Planning involved as many as 22 countries.

The exercise is an annual affair that began in 2012 under the Obama administration, but took on a different coloration after the Iranian-backed attack on Saudi Arabia’s main oil-processing facility on Sept. 14. When Trump and the Saudis backed away from direct military retaliation, military exercises took on heightened significance. Vice Admiral Jim Malloy, commander of U.S. Naval Forces Central Command, declared on a visit to Riyadh on Sept. 29 that “engaging and operating closely with regional counterparts is essential to maintain deterrence.”

But at this moment when U.S. policy in the Middle East appears to be in growing disarray, the question emerges: Who will be the guarantor of security for the vast quantities of hydrocarbons produced and shipped from the region? And there are now ample signals that Russia wants to step into a role as part of its expanding influence in the region.

One of the clearest indicators came last month when Iran—yes, Iran—announced through its official media that it would soon participate in joint naval exercises with Russia and China. Yes, China.

Those reports came soon after the United States, in the aftermath of the attacks on Saudi Arabia, said it would be sending a few hundred American troops to bolster the kingdom’s defenses. That augmentation has since been increased to 3,000 U.S. troops. But the Iranian announcement was not merely reactive and should not have come as a surprise, at least where the Russian-Iranian connection was concerned.

“ At the beginning of this month, Moscow confirmed preparations for a joint naval exercise with China and Iran in the Indian Ocean. ”

Already at the end of July, according to Jane’s Navy International, Rear Admiral Hossein Khanzadi, commander of the Iranian Navy, and Russian Navy chief Admiral Nikolai Anatolevich Evemenov “signed a memorandum of understanding” to “expand bilateral ties.”

At the beginning of this month, Moscow confirmed preparations for a joint naval exercise with China and Iran in the Indian Ocean. At the Valdai Discussion Club held from Sept. 30 until Oct. 3 in Sochi, Russian Foreign Minister Sergey Lavrov said flatly “We, the People’s Republic of China, and Iran are preparing naval drills for fighting terrorists and pirates in this part of the Indian Ocean.”

The limited scope reflected China’s caution. Analysts told the South China Morning Post in September, just after the Iranian announcement, that Beijing probably would not send anything more than a few ships from its anti-piracy squadron, which has been in place off the coast of Somalia for years. The last thing it wants is to get caught between Washington and Tehran.

But as military analyst Song Zhongping told the Morning Post, the Chinese “escort fleet” off the African coast is looking to extend its reach into the northern Indian Ocean and the Strait of Hormuz, waterways “important to China’s oil lifeline in the Middle East.”

“ We are trying not to hurry up. ” — Andrei Fedorov, Russian political analyst

So, is the planned joint exercise part of a “preemptive defense” strategy against a possible U.S attack on Iran? Although the message is carefully calibrated, even discussion of such exercises is a show of support for Tehran at a time when Iran is reeling under the U.S policy of “maximum pressure,” pushing it toward economic isolation worldwide.

The planned joint exercise does not guarantee Beijing and Moscow would side with Tehran if Iran is attacked by the U.S. or Israel, but at a minimum it suggests that possibility. Iran wants to show that it may be isolated economically but not politically or militarily. And China and Russia want to show their solidarity while taking, for the moment, minimum risks.

The Russian foreign minister’s announcement at the Valdai conference at the beginning of the month “was at the request of Iran,” Andrei Fedorov, director of the Center for Political Research and Consulting in Moscow, told The Daily Beast, “but we are trying not to hurry up.”

For the moment, Russian President Vladimir Putin’s attention is fixed on the situation in Syria, which he will discuss with Turkish President Recep Tayyip Erdogan at a summit meeting Tuesday.

Over the medium and long term, however, if China, Russia and Iran continue to develop their ties to protect their strategic interests in the Indian Ocean they will form a powerful trio, with Russia taking the lead.

The concept of collective security in the Persian Gulf was introduced by Moscow in July this year. The concept stipulates organizing an international conference on security and cooperation in the Gulf, which will later lead to creating a security and cooperation organization in the region.

China endorsed that overall concept in July. “We welcome the Russian initiative," said Chinese Foreign Ministry spokesperson Hua Chunying, according to a report by the Russian news agency TASS. “We would also like to boost cooperation, coordination, and communication with all the corresponding parties,” Chunying said.

“ China also sees strategically located Iran as an important link in the One Belt One Road initiative that could connect it to Europe. ”

China and Russia, both of them permanent members of the United Nations Security Council, have continued business with Tehran, defying the sanctions re-imposed by the Trump administration, and both the countries are set to deepen their involvement in Iran’s energy and infrastructure sectors.

Despite Washington’s unilateral sanctions on Iranian oil imposed in May, China imported more than 900,000 metric tons of crude oil from Iran in July, up more than 8 percent from the month before, according to China’s General Administration of Customs.

China also sees strategically located Iran as an important link in the One Belt One Road (OBOR) initiative that could connect it to Europe. China and Iran agreed to bolster bilateral defense-military cooperation in 2016, and the two countries discussed a road map for the China-Iran comprehensive strategic partnership this August, when Iran Foreign Minister Mohammad Zarif paid a visit to his Chinese counterpart Wang Li.

China is the world’s largest importer of crude oil, and Saudi Arabia, which supplies the Chinese with a million barrels a day, has now become Beijing’s second largest crude oil supplier, after Russia. But Beijing would like to diversify by finding a way to expand its importation of Iranian oil.

Thanks to president Trump’s manufactured crisis in the Persian Gulf after his unilateral withdrawal from the Iran nuclear deal and the policy of economic strangulation against Tehran, China, Russia and Iran are drawing closer as strategic partners.

—Christopher Dickey also contributed to this article.