On October 5, 2017 (August 16, 2017 in the lunar calendar), the Saudi King visited Russia for the first time since the establishment of diplomatic relations. Late at night on October 4 local time, Saudi King Salman began his visit to Russia. This was the first time a Saudi King has visited Russia since the establishment of diplomatic relations between the two countries. The Russian Federation President Vladimir Putin held a formal meeting with him on October 5, 2017, and the two sides reached consensus on a series of cooperation issues; among them, cooperation in arms attracted particular attention. According to local Russian media reports, the two countries reached a military purchase agreement of at least US$3 billion, including advanced equipment such as S-400 air defense missiles. On October 5, local time, in Moscow, Russia, Russian President Vladimir Putin met with King Salman of Saudi Arabia. Arab Satellite TV reported: "Saudi Arabia has agreed to purchase advanced S-400 systems." Sky News Arabic channel also reported similar news. Russian Ambassador to Saudi Arabia Oleg Ozerov said in an interview with Sputnik News Agency earlier that Saudi Arabia hopes to purchase the S-400 air defense missile system from Russia. The S-400 "Triumph" is Russia's most advanced long-range air defense missile system and was launched into troops in 2007. It is used to intercept aircraft and cruise missiles, and has anti-missile capabilities, including medium-range missiles. According to public information, the S-400 has a maximum range of 400 kilometers and can destroy high-altitude targets as high as 30 kilometers. In addition, the Russian Defense Export Corporation signed a contract with Saudi military enterprises to authorize Saudi Arabia to produce Kalashnikov assault rifles AK-103 and various bullets in the country. Alexander Mikheyev, general manager of the Russian Defense Export Corporation, and Ahmed Khatib, chairman of the Saudi Military Industry Corporation, signed the contracts respectively. At the same time, Saudi Arabia may also purchase aircraft and helicopters from Russia, but negotiations are still in progress and have not yet been finalized. The above arms sales and cooperation news came on the same day that Russian President Vladimir Putin met with Saudi King Salman. Russian Foreign Minister Lavrov hailed Salman's visit as a "historic moment"; on October 5, Putin called it a "milestone event" and believed it would promote relations between the two countries. Salman called Russia a "friendly country." Previously, according to news from the Kremlin website and Russia Today's Russian News Agency on the 5th, Putin and Salman held separate talks and expanded meetings of high-level delegations on the same day. The two countries signed a total of 14 cooperation documents, covering military industry, oil and gas, nuclear energy, transportation, communications, agriculture and other fields, and discussed Middle East issues such as the situation in Syria, Iraq, Libya and Yemen. On the Syria issue, Salman said that Saudi Arabia positively evaluated Russia's efforts, supported the Astana talks on Syria, and believed it necessary to maintain Syria's territorial integrity. Saudi Arabia will seek a solution to the Syria issue in accordance with UN Security Council resolutions and the results of the Geneva peace talks. On the issue of oil and gas, Salman said that Saudi Arabia seeks to continue cooperation with Russia to achieve stability in the international crude oil market. Has the relationship between the two countries cleared the fog? Russia and Saudi Arabia have always had differences in many fields. There are even sharp contradictions between the two sides on the issues of Syria and Iran. On the issue of oil prices, the two sides are also tit-for-tat. Some media believe that the Saudi King's visit to Russia marks a new level in relations between the two countries. According to China Radio International, Saudi Arabia has long supported the Syria opposition and called for the resignation of Syria President Assad. There is an irreconcilable contradiction between Saudi Arabia and Iran, while Russia has always supported Assad and regards Iran as an ally. Saudi Arabia itself is a traditional ally of the United States in the Middle East. However, analysis pointed out that with the shift and contraction of the United States 'strategic focus and the continuous deterioration of geopolitics in the Middle East, Russia took the opportunity to intervene in Middle East affairs. In Syria, quick actions were taken to cut the Gordian knot and successive results were achieved. It is exerting increasingly important influence, which is one of the reasons why Saudi Arabia moves closer to Russia and relations between the two countries continue to heat up. In addition, as the battlefield situation in Syria gradually becomes clearer, all parties have their own considerations on how to allocate their interests in Syria in the future. Saudi Arabia's main starting point in Syria is actually what was previously called the Nusra Front. However, Russia may achieve a "monopoly" in Syria in the future. How can Saudi Arabia retain its agent and share in Syria requires a transaction with Russia. At the same time, it should also be noted that Saudi Arabia has always been a traditional Middle East ally of the United States. Since the period when the United States and the Soviet Union competed for hegemony, Saudi Arabia and Russia have had constant differences. Now that the two sides are getting closer and closer, what are the prospects? Li Shaoxian, an expert on Middle East issues and dean of the China Institute of Arab States Studies at Ningxia University, said that the two sides are currently coming together because of interests, but at the same time there are great constraints. The relationship between Saudi Arabia and Russia cannot be viewed too far. Saudi Arabia will not secede from the United States. The biggest constraint on Saudi Arabia's relations with Russia is the alliance between Saudi Arabia and the United States. Saudi Arabia will never tolerate damage to its alliance; For Russia, Russia may value Iran more. Now Saudi Arabia and Russia are moving together, and the Saudi king is visiting Russia for the first time. This is indeed historic, but there are certain limitations on the development of relations between the two sides.