Defense Secretary James Mattis on Tuesday announced that he plans to travel to Macedonia this weekend to “make very clear” American stands against Russian aggression as the nation considers its invitation to join NATO.

“I’m going to go out and see our Macedonian friends on a rather swift journey over there and back,” Mattis told reporters at the Pentagon. “I’m going there to make very clear we stand with the Macedonian people.”

Macedonia, formerly part of Soviet ally Yugoslavia, was formally invited by NATO in July to start accession talks, which Russia has opposed. Moscow officials have said the nation could become “a legitimate target” if relations between NATO and Russia deteriorate further.

Mattis said he wants “to make sure that they know that we believe it should be our Macedonian friends charting their country’s future, and not outsiders.”

The Pentagon chief previously met the Macedonian defense chief at a meeting of Balkan defense heads in July in Croatia, days before the one-on-one meeting between President Trump and Russian President Vladimir Putin in Helsinki.

Following Trump’s meeting, the president questioned the value of defending newest NATO member Montenegro against a Russian attack. Montenegro sits northwest of Macedonia and was also previously part of Yugoslavia.

“Montenegro is a tiny country with very strong people. … They’re very aggressive people. They may get aggressive. And, congratulations, you’re in World War III,” Trump said while on Fox News’s “Tucker Carlson Tonight,” appearing to question the purpose of NATO’s Article 5.

Article 5 states that an attack on one member is an attack on all. It has only been invoked once, following the Sept. 11, 2001 terror attacks on the U.S.

Montenegro, in turn, has accused Russia of trying to prevent the country from joining NATO and of an assassination attempt on its prime minister.

Mattis said he’s “concerned” that Russia might also try to interfere with Macedonia in a similar fashion.

“The kind of mischief that Russia has practiced from Estonia to the United States, from Ukraine and now to Macedonia … it’s always beyond the pale, as far as I’m concerned.”