The U.S. will take over the Baltic air policing mission next month in an effort to guard airspace over Lithuania, Latvia and Estonia, U.S. European Command said Wednesday.

Air Force F-15C Eagles from the 48th Fighter Wing, RAF Lakenheath, United Kingdom, will lead the next NATO Baltic Air Policing rotation at Šiauliai Air Base, Lithuania, EUCOM said.

The F-15s, acting as a quick reactionary force, will surveil the air until the end of the year.

The US last was part of the Baltic Air Policing mission in 2014, the same time Russia annexed Crimea.

The Air Force’s obligations to the Baltic countries — which lack capable air forces — came under scrutiny last year after then-President-elect Donald Trump criticized the U.S. for acting as the backbone to the NATO alliance, and left the door open on whether he would defend countries he deemed as not properly contributing.

Then-Air Force Secretary Deborah Lee James in December touted the policing mission as critical, given that the Russian air force conducted a flurry of unsafe maneuvers and flybys near or over the unprotected airspace.

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James said the German air force, overseeing the mission at the time, reported “more than 30 scrambles between the end of August and the start of November this year, intercepting Russian aircraft flying near civilian air routes with their transponders turned off.”

The former secretary said there were no plans for the U.S. to discontinue assistance in 2017, thanks in part to European Deterrence Initiative (formerly the European Reassurance Initiative), “to reflect that our presence …does more than reassure.”

“I’ll just repeat what [Baltic nation leaders] have told me, they certainly view these actions as a major deterrence,” James said at a talk hosted by the Atlantic Council in Washington, D.C.

Since the annexation of Crimea, the Air Force has deployed aircraft, from F-15s to A-10s ground-attack aircraft, as part of a regular rotation of assets meant to deter Russian aggression on the continent.

The U.S. is replacing Poland’s F-16s for the latest mission, according to NATO.

Additionally, six F-15Cs from the Louisiana and Florida Air National Guard deployed as the 159th Expeditionary Fighter Squadron to Keflavik, Iceland, to conduct an air surveillance mission in support of NATO, according to U.S. Air Forces Europe.

The aircraft arrived in Iceland on Aug. 23, USAFE said.

The US has conducted the NATO mission in Iceland annually since 2008.