Poland on Wednesday signed off on a deal to buy Raytheon’s latest Patriot air and missile defense system, a move to upgrade the country’s air defenses that took years to negotiate.

The Polish government signed a letter of offer and acceptance with the United States for the first phase of Warsaw’s Wisla program, a two-part procurement of the medium-range, integrated air and missile defense system, according to a statement from U.S. defense contractor Raytheon.

The system includes Northrop Grumman’s Integrated Air and Missile Defense Battle Command System (IBCS), and Lockheed Martin’s Patriot Advanced Capability-3 Missile Segment Enhancement missiles, according to separate company statements.

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The deal, which will eventually include eight batteries, is expected to be worth nearly $6 billion.

The new Patriot deal is likely to stoke Russia-NATO tensions. Russian President Vladimir Putin has said Moscow views the missile defense systems in eastern Europe as a “great danger.” Putin has also threatened that Russia will be forced to enhance its own missile strike capability.

The sale has long been in the making, with Poland first intending to buy the Patriot system in 2014. A new Polish president in 2015, however, threw those plans into question as the government wanted a look at other possible systems.

The deal was also slowed by Poland’s insistence that domestic industrial companies be involved in at least 50 percent of the system’s construction.

After some additional negotiations, which included the incorporation of Northrop’s still-in-development IBCS, in July, the U.S. and Polish governments signed a memorandum to buy Patriot missiles.

Fourteen countries, including six NATO partners, already own Patriot systems, and more nations intend to purchase the missiles.

Romania in November signed a letter of offer and acceptance to buy Patriot missiles to help protect its airspace, and Sweden has submitted a formal request for the system.