“The S-400 is among the most advanced air defense systems available, on par with the best the West has to offer.”

On Wednesday, Turkish president Recep Tayyip Erdogan announced Turkey bought a Russian anti-aircraft missile system. The announcement came after months of public sparring between Turkey and the U.S. over the potential purchase of the defense system which is not compatible with NATO’s systems.

“Turkey has already bought S-400 defense systems. It is a done deal. I hope these systems will be delivered to our country next month,” Erdogan said, as Al Jazeera reported.

Previously, President Donald Trump condemned Turkey’s plan to purchase Russia’s air defense system given that both the U.S and Turkey are members of NATO. The U.S. and other NATO members fear that Russia could spy on NATO aircraft through its S-400 system.

The U.S. claims it is worried about Russian technicians having access to the F-35, the U.S.’ most expensive fighter jet ever.

“We do not want to have the F-35 in close proximity to the S-400 over a period of time because of the ability to understand the profile of the F-35 on that particular piece of equipment,” Under Secretary of Defence Ellen Lord told reporters on Wednesday.

Washington has threatened to withhold the sale of F-35’s to Turkey, if Turkey bought the S-400 system from Russia. Acting U.S Secretary of Defense Patrick Shanahan last week warned that Turkey could not have Russia’s S-400 and U.S. F-35s at the same time.

After Turkey’s announcement of the S-400 purchase, the U.S. said it was postponing the training for Turkish pilots on F-35 fighter jets. The U.S. also threatened Turkey with further, harsher economic sanctions.

Turkey had decided to buy 100 units of F-35s and had invested a lot in the fighter jet program. Several Turkish companies produce 937 the jet’s spare parts.

In recent years, Turkey has forged closer ties with Russia following worsening relations with the U.S. and Europe. But Reuters reported that Erdogan said he hoped to resolve the situation with the U.S. through phone diplomacy ahead of a meeting with President Trump at the end of June.

The Turkish newspaper Yeni Safak reported that in response to the U.S. halting the sale of the F-35s, Turkey was preparing “plans B, C or D”  which involve developing a “homegrown” fighter jet or turning to China or Russia for talks regarding their fighter jets.

“In addition to the Russian SU-57s, the security bureaucracy is also investigating China’s J-31s. Moreover, these aircraft stand out with their low cost compared to the F-35s, each of which amounts to approximately $ 100 million. Even though the F-35 have already arrived in Ankara, considering this is directly connected to the United States each command aircraft will accommodate serious security risks for Turkey. That’s why no one regrets the ‘loss’ of the F-35s,” the pro-government paper wrote.

What is S-400?.

The S-400 “Triumph” (NATO calls it “Growler”) is one of the most advanced surface-to-air missile defense systems. In 2017, The Economist described the S-400 as “one of the best air-defense systems currently made.” It has a range of 250 miles, and one triggering of the S-400 system can shoot down up to 80 objects simultaneously.

According to the BBC, Russia claims its S-400 can hit a wide range of targets including low-flying drones to aircraft at various altitudes and long-range missiles.

The defense system was developed in the 90s and was first put to use in the Russian military in 2007. It is regarded as one of the best anti-ballistic missile systems worldwide.

“The S-400 is among the most advanced air defense systems available, on par with the best the West has to offer,” said Siemon Wezeman, senior researcher with Stockholm International Peace Research Institute’s (SIPRI) arms transfers and military expenditure program.

“Its radars and other sensors, as well as its missiles, cover an extensive area – the radar has a range of at least 600km for surveillance, and its missiles have ranges of up to 400km,” Wezeman told Al Jazeera.

“It’s precise and it manages to track a very large number of potential targets, including stealth targets.”

The S-400 is also attractive due to its high mobility, it can be activated, fired up and removed within minutes.

Besides Turkey, other countries such as China, Saudi Arabia, Qatar, and India have expressed interest in purchasing the S-400 system.

Last year, India inked a $5.43 billion deal to buy the S-400 system, snubbing a threat by Washington to New Delhi.

Why is S-400 Triggering Concern From the US and NATO?

The U.S. and NATO fear Turkey’s interest in Russia’s anti-aircraft missile system is politically and technically motivated as Kevin Brand, a military analyst at the Council on Foreign Relations, explained to Al Jazeera.

“In the technological sense, the S-400 would certainly be a step forward [for Turkey], but it’s not necessarily in the best interest of NATO to have that weapons system integrated within its broader architecture,” Brand said.

“When you look at the Russian S-400 system, especially in a NATO structure, there’s a scale of difficulty when integrating it into the bigger defense system,” Brand told Al Jazeera.

“If you take it as a very benign situation, the simplest scenario is that its data might not be able to be incorporated into the defensive architecture that is currently used by NATO. That’s probably the best worst-case scenario.”

NATO is heavily interdependent on several systems working together in a larger network.

But the U.S. and NATO are also worried that Russia might have ill intentions behind the sale of its S-400, Brand explained to Al Jazeera.

“What kind of contracts would be in place with Russian technicians taking care of the S-400, for example, would Russian maintenance personnel have access to [NATO] data?

“The worst-case scenario is that there might be vulnerabilities associated with that system that could be exploited by a potential adversary.

“Plugging it in could potentially actively compromise your own defensive network.”