The US stopped receiving more Turkish pilots for an F-35 training program, Reuters reported on Friday. The same day Russia said it had not received a request from Iran to purchase the Russian-made S-400 air defense system. Both reports relate to Russia’s S-400 sales and its use of the advanced defense system as a form of Moscow’s diplomacy in the Middle East. Turkey agreed to purchase the system and is supposed to take delivery of it this month as part of a growing Turkey-Russia alliance. This has angered the US.Not long ago Russia and Turkey were on different sides of the Syrian civil war, with Russia supporting the Syrian regime of Bashar al-Assad and Turkey backing the rebels. But the two countries have grown closer in recent years, with energy deals and also working closely on ceasefires in Syria. Russia understood that selling its air defense system to Ankara was a game changer. Turkey is a member of NATO and is also part of the US F-35 warplane program. Yet Washington has said that the S-400 purchase could scupper the F-35 relationship. Turkish pilots began training last year, but in late May reports emerged that the US might end additional training. Reuters says that is exactly what has happened.The US stopped accepting additional Turkish pilots. Two US officials said that the decision could be reversed and that it only affects upcoming groups of Turkish pilots and maintenance crews. “There had not yet been a formal decision to halt the training of the Turkish pilots and maintenance crews now at Luke Air Force Base in Arizona.” But that could be the next step. Four pilots are currently being trained.The US is unsurprisingly angry. The S-400 is built by a US adversary and having the same system in a country with the F-35 could enable crews to learn how to confront the advanced US plane. This would affect the whole point of having an advanced aircraft that is supposed to avoid systems like the S-400.Turkey’s President Recep Tayyip Erdogan has repeatedly said that Turkey will not cancel its deal. He reiterated his claim on June 4. “We have made a deal and our determination is evidence. It is out of the question for us to walk away from this,” he said. He also indicated Turkey would consider purchasing the US Patriot air defense system, a deal that was discussed in December 2018. Meanwhile the US has discussed a working group to look at the S-400 controversy with Turkey, but the US has also said there is no change in its view that the purchase is unacceptable.On May 30, the first reports emerged that Russia had turned down a request by Iran to acquire the S-400. Bloomberg noted that Iran’s Foreign Minister had visited Moscow on May 7 and that Moscow had rejected Iranian interest in the system. The US and Iran escalated tensions in May as the US accused Iran of threats in the Gulf. Russia didn’t want more tensions in the region.Russia’s policy of reducing tensions is part of a wider policy in Syria. It has agreed to meet with the US and Israel for a trilateral meeting this month. It is also seeking to reduce tensions in Syria amid increased clashes between Israel and Syria. There have been four incidents, including airstrikes by Israel, reported by Syrian media between May 27 and June 6. Russia doesn’t want another conflict in Syria because its Syrian ally is already fighting a Syrian rebel offensive in Idlib. Russia and Turkey signed a ceasefire agreement in Idlib in September 2018, but that agreement is quickly becoming tatters with massive clashes between the Syrian army and rebels there. This is important for Russia, because Russia has a military base in Latakia, near Idlib. So any tensions in the south between Israel and Syria affect Moscow’s aims for the region. This also relates to wider tensions between the US and Iran and Israel and Iran. From Moscow’s perspective a regional war between the US and its allies and Iran would be a disaster.Although Russia may be pleased that its S-400 deal with Turkey is going through, it appears to be less interested in cementing an alliance with Iran using the S-400. Deputy Prime Minister Yury Borisov told Sputnik on Friday that Moscow has not received a request from Iran for “delivering its S-400 air defense systems.” Turkey’s TRT and other media reported this Russian claim. Borisov met with a Syrian delegation on June 5, according to Syrian media, which shows that Russia’s main interest is Syria at the moment, not Iran. Borisov called the reports about Iran and the S-400 “fake news.”The Russia response to claims about Iran’s interest was reported on Fars News in Iran. The reports noted that Moscow had said there had been no offer from Iran.If the Russian response to rumors about Iran’s interest are accurate it still shows that across the region the issue of the S-400 and Russia’s role is of paramount importance. The rumors about Iran’s interest may have been designed to test the US and Israeli response. Iran isn’t the only other country that wants the system in the Middle East. On May 15, Russia’s TASS news agency reported that Iraq was seeking to purchase the system. The US is closely involved in training and equipping Iraqi forces, which would mean another US ally in the region was becoming part of the S-400 club. Last year, CNBC reported that Saudi Arabia, Qatar, Algeria, Morocco and Egypt had all discussed the S-400. Saudi Arabia, Qatar, Egypt and Morocco are all US allies, with Saudi Arabia a key military partner and Qatar home to a US base. The US has also supported Egypt’s military since the 1980s. Russia’s S-400 has the potential to both embarrass Washington and upend the traditional US alliance system in the region.