We are committed to taking all necessary legislative action to ensure this is the case. Turkey is an important partner in the F-35 program, but it is not irreplaceable.

Abandoning the F-35 will have severe consequences for Ankara. Turkey has already invested more than $1.25 billion in the F-35 program, and that will be squandered. It will not receive the more than 100 F-35s it planned to purchase, and it will be forced to settle for a less-capable fighter aircraft that will not arrive for many years.

Turkish companies that produce parts for the F-35 will see their orders dry up completely. The country’s F-35 engine maintenance, repair, overhaul and upgrade facility will see all its work go to other facilities in Europe. President Erdogan’s hope to make the Turkish defense industry a pillar of economic growth for the future will be dashed.

We do not seek to harm our Turkish friends. Indeed, we hope it will be possible to enhance American-Turkish cooperation on Syria, the Black Sea, counterterrorism and other issues of mutual concern. We seek only to protect the F-35 program and the capabilities of the NATO alliance, including allies like Turkey.

We understand that Turkey has a relationship of necessity with Russia — on Syria , energy, agriculture, tourism and more. If President Erdogan walks away from the S-400, Mr. Putin may retaliate in one or more of these areas. In that unfortunate event, we commit to do all we can to assist Turkey as it weathers the storm.

Paying tribute to the Kremlin with the purchase of the S-400 is not in Turkey’s interests. Mr. Putin is not an ally of Turkey any more than the Soviets or the czars. His aggression in Georgia, Ukraine and Syria has made Turkey less safe. Now Mr. Putin is trying to divide Turkey from the West with the S-400s.

If he succeeds, what little regard he has for Turkey’s interests will shrink further. The more isolated Turkey is from its allies, the more power Mr. Putin will have in the relationship: Russia does what it can, Turkey suffers what it must.

Mr. Putin fears and respects a Turkey strategically anchored in the West and committed to NATO. We hope President Erdogan will choose that future for Turkey by rejecting Mr. Putin’s divisive S-400 ploy, meeting its air defense requirement with the Patriot system and moving forward as a critical partner in the F-35 program.

Jim Inhofe is a Republican Senator from Oklahoma. Jack Reed is a Democratic Senator from Rhode Island. Jim Risch is a Republican Senator from Idaho. Bob Menendez is a Democratic Senator from New Jersey.

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