Turkey took its first deliveries of the Russian S-400 air defense system on Friday, a development likely to trigger US sanctions on Ankara and escalate tensions between Turkey and its Western partners in the NATO military alliance.Why does Turkey need the Russian defense system? Ankara sees the system as a strategic defense requirement as it faces security threats from its southern borders with Syria and Iraq. It says that when it made the S-400 deal with Russia, the United States and Europe had not presented a viable alternative. Turkey has said the Russian system met its expectations in terms of price and technology and it hopes to cooperate with Russia in the development of the next generation S-500 missile systems, benefiting from technology transfer and co-production. According to Ankara, the S-400s do not pose a direct threat to the US military. It has proposed setting up a technical working group that could include NATO, to address US concerns about the potential impact of the system on US F-35 jets. This proposal has so far been left unanswered, Turkey says.Why is the United States opposed to the deal? The United States says the Russian missile system is not compatible with the NATO defense network and that it could pose a threat to its Lockheed Martin F-35 stealth fighter jets, an aircraft that Turkey is helping to build and planning to buy. Turkey’s purchase also contravenes US legislation, known as Countering America’s Adversaries through Sanctions Act (CAATSA), which requires Washington to impose sanctions on countries which buy military equipment from Moscow. The missile deal further complicates US policy in the Middle East by heightening tensions with Ankara at a time when Washington has been exerting pressure internationally to isolate Iran by blocking Tehran’s oil exports. Turkey had been a major buyer of Iranian oil, but has halted purchases so far. The United States and Turkey are also at odds over the Syrian conflict and other issues.What has the United States offered as an alternative? In trying to persuade Turkey to give up the Russian missiles, the United States offered to sell its Raytheon Co. Patriot missile defense systems. Turkey’s defense minister has said Turkish and US officials were working on price, technology transfer and joint production issues on the latest US offer, made in late March. However, no agreement has yet been reached. Several NATO allies have also provided Patriot missile batteries to protect Turkey’s southern borders during the conflict in neighboring Syria.What consequences will the deal’s completion have? Washington has repeatedly said Turkey will face “real and negative consequences” if it proceeds with the acquisition, including suspension of procurement and industrial participation in the F-35 program and exposure to sanctions under CAATSA. If the United States removes Turkey from the F-35 program, and imposes sanctions on its NATO ally, it would be one of the most significant ruptures in recent history between the two nations. However, Erdogan has repeatedly voiced confidence in his rapport with US President Donald Trump, who has expressed sympathy over Turkey’s situation. At a meeting with Erdogan in June, Trump blamed former President Barack Obama’s administration for failing to help Turkey acquire Patriot missiles as an alternative to the S-400s. In theory, the president has the authority to withhold or delay CAATSA sanctions. However, US officials have said that the Trump administration still plans to impose the sanctions on Turkey and remove it from the F-35 program.