Obama’s just completed his trip to Vietnam, the first of his presidency.

The US President discussed trade and investment, economic and energy development, human rights and democracy (the main area of disagreement in the joint presidential news conference), academic exchanges and humanitarian assistance, and of course regional security issues (i.e., China).

It was impressive to see how warmly the president was greeted, both in his public speeches and during his motorcades, when many younger people, eager for more political and commercial freedom, cheered his appearance.

For the readers of this website, perhaps the most important event was Obama’s announcement that the United States would remove the last prohibitions against Vietnamese purchases of U.S. weapons as well as expand other military cooperation.

Vietnam now will be subject to the same arms transfer executive and congressional branch rules that apply to all foreign military buyers of the United States. The Vietnamese welcomed the measures as signaling how their relationship had finally been “normalized.”

For both governments, the move was clearly aimed at strengthening Vietnam’s ability to counter further Chinese encroachments in the disputed maritime territories of the South China Sea.

U.S. official worry that, if Beijing continues its present coercive tactics and regional militarization, the vitality of this critical seaway will be threatened.

Obama insisted that the Pentagon would continue to send ships and planes through this vital waterway and would defend the principles of peaceful resolution of disputes, freedom of commerce, adherence to international law, and respect for other norms sometimes contested by Beijing.

In signaling U.S. willingness to expand the transfer of arms to Vietnam, U.S. policy makers aim to discourage such Chinese behavior directly and by showing Beijing how assertive Chinese policies are driving Beijing’s neighbors to align with Washington against China.

According to media reports, including press briefings by U.S. officials, before the president’s departure for Hanoi the Obama administration remained undecided on whether to lift the arms sale ban.

What may have finally tipped the scales in favor of removing all restrictions was the aggressive flying by Chinese fighter jets a week before the visit to maneuver within about 15 meters of a U.S. Navy reconnaissance plane.

Chinese opposition to U.S. surveillance patrols is well known, but these flights, as well as U.S. maritime reconnaissance patrols, occur outside Chinese territory over international waters. Since the Chinese harassment closely resembles recent incidents where Russian warplanes flew recklessly near U.S. patrols in international waters, the Pentagon may have lobbied for renewed arms sales as a means of signaling to Moscow as well as Beijing that such actions have costs.

Developing this line further, besides deterring Chinese adventurism, the new U.S. approach toward Southeast Asian arms sales provides an opportunity to weaken the Beijing-Moscow alignment.

In addition to its well-known security ties with China, Russia also tries to maintain military cooperation with the member countries of the Association of Southeast Asian Nations (ASEAN) through bilateral dialogue and drills as well as through multilateral structures.

Of note given Obama’s repeal of the arms embargo on Vietnam, has become a more important market for Russian weapons sales in recent years.

At a time when Rosoboronexport, the state corporation that oversees Russia’s foreign weapons sales, is thinking of skipping the Farnborough International Airshow since the European arms market is “not interesting” to the Russian defense industry due to sanctions and other restrictions on defense collaboration.

Whereas ASEAN states bought only six percent of Russia’s arms exports in 2010, the figure increased to 15 percent in 2015. From 2007 through 2014, Russian weapons sales to the Asia-Pacific region and other Asian countries—including hundreds of tanks, warplanes, helicopters, armored vehicles, and self-propelled guns, as well as thousands of missiles–amounted to more than $30 billion.

Rosoboronexport, that the “successful use of Russian weapons on large- scale counterterrorist operations” will boost sales even further in coming years—such as of the Kalibr (NATO reporting name: SS-N-27 Sizzler, or “Club” for the export version) cruise missiles, and the Russian Naval platforms that launched them against Syria.

Indonesia is anticipated to buy advanced Russian Su-35 Flanker-E (export-version) fighters, which only China has purchased so far, to supplement the Su-27 and Su-30s already in its fleet and replace its aging U.S.-made F-5E/F fighter planes.

Rosoboronexport also expects the Indonesian Marine Corps to buy more Russian-made BMP-3F infantry fighting vehicles.

Rosoboronexport also hopes to sell more advanced fighters to the Royal Malaysian Air Force, building on the earlier $900 million sale of 18 Su-30MKM ([NATO reporting name: Flanker-H) fighters. Russian Helicopters continues to service the combat helicopters Laos purchased from Russia in the 1990s.

In pursuit of a June 2015 contract, Russia has already planned to deliver three Yakovlev Yak-130 (Mitten) combat-ready trainer planes and associated equipment to Myanmar (Burma) by the end of the year. Bangladesh has also purchased some of the Yak-130 and Vietnam has expressed interest in the plane as well.

Vietnam has arguably been Russia’s closest strategic partner in Southeast Asia.

In 1979, following the Chinese invasion of Vietnam, Moscow established a naval base in Cam Ranh Bay, which it now uses to refuel Tu-95MS Bear nuclear-capable strategic bombers that engage in long-range patrols over the South and Central Pacific, including the U.S. West Coast and the U.S. territory of Guam—leading to Washington protesting to Hanoi.

Despite Beijing’s objections, Gazprom is drilling in maritime zones disputed by both countries.

Thanks to its growing military budget and its strained relations with China, Vietnam has become a major Russian weapons buyer.

Russia also provides most of Vietnam’s military training, though India, which has experience operating the export version of Russian weapons, also provides training. A $3-billion contract signed in 2009 to equip the Vietnam’s People’s Navy (VPN) with six Type 636 Kilo-class diesel-electric submarines, armed with torpedoes, mines, and Klub supersonic cruise missiles that can hit naval and coastal targets, should be completed this year.

Vietnam’s People’s Air Force should have three dozen advanced Su-30MK2s by the end of this year and is considering buying the Su-35S, all designed to replace its aging fleet of Soviet-era MiG-21, Su-22, and Su-27 fighters. The VPN is acquiring a half-dozen stealthy Project 1166 Gepard 3.9/Dinh Tien Hoang-class light frigates, armed with sub-sonic Kh-35E anti-ship missiles, to add to its flotilla of Russian-designed Project 12418 Molniya missile-armed Fast Attack Craft, fast patrol boats, corvettes and frigates that are optimized for littoral combat.

Most of the Army’s tanks, helicopters, and other equipment also comes from the Soviet Union or Russia, and the Army is now considering buying T-90 main battle tanks to supplement its hundreds of T-72s and replace its T-55s. Vietnam produces some of these weapons systems and their armaments, like the Kh-35 anti-ship missile, under license from a Russian manufacturer.

At times, Russia has found it challenging to manage the tensions between China and the ASEAN states.

Whereas Beijing prefers to address its territorial disputes over the South China Sea and other differences with the ASEAN states unilaterally, or at least without the intervention of non-ASEAN members, many of the latter have been seeking to internationalize their disputes by drawing in outside powers that could balance China’s superior economic and military power over the ASEAN countries.

Whereas the United States—sometimes joined by Japan, India, or Europe–has pursued this line to help avoid military conflicts or other coercive action that could threaten the usability of this vital waterway, Russia has traditionally sought to distance itself from any regional disputes involving China and Southeast Asian countries to avoid antagonizing one of the parties.

Nonetheless, Moscow has made moves that could be seen as favoring China or the other parties.

At the April 2016 the Conference on Interaction and Confidence Building Measures in Asia, Russian Foreign Minister Sergei Lavrov joined Chinese Foreign Minister Wang Yi in demanding that all outside powers refrain from interfering in differences among Asian powers, such as regarding the rival territorial claims in the South China Sea, which they insisted should be resolved through direct talks among the parties to the dispute.

On the other hand, the communique issued at the ASEAN summit in Sochi titled toward the ASEAN position by calling for more rapid adoption of a “code of conduct” and more dialogue on regional security issues at the East Asian summits, which include Russia as well as China and the United States.

The U.S. decision to expand arms sales to Vietnam, and presumably other Southeast Asian states, provides an opportunity to tip Moscow off this balance.

In the near term, these countries will want means to monitor and discourage Chinese adventurism—which would mean buying U.S. maritime patrol planes and helicopters, coastal radars and reconnaissance UAVs, and fast patrol craft and other littoral vessels.

But these purchases could expand over time and, in some cases, signify a decision by these countries to fundamentally upgrade their security ties with the United States to have better support in Washington against China.

We have already seen this in the efforts of some countries to join the U.S.-led Trans-Pacific Partnership, despite Beijing’s objections.

The U.S. government will need to expand its economic and other assistance for these states to achieve U.S. goals and to enable Asian partners to execute their pivot plans.