BRUSSELS -- Over the past four months, NATO has been flexing its muscles. It has deployed thousands of soldiers and heavy weapons along Russia’s border.

The treaty alliance’s annual report calls it the largest reinforcement of collective defense since the Cold War -- and a response to Russia’s invasion of Crimea and the destabilization of Ukraine.

The build-up is meant as both a warning, and a deterrent.

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“What NATO does is defensive, it is proportionate and it is measured because we don’t want a new Cold War,” NATO Secretary General Jens Stoltenberg told CBS News after he presented the annual report in Brussels on Monday. “We don’t want a new arms race.”

But for Russia to take the deterrent seriously, NATO has to remain united. The basic pact must hold; if one country in the alliance is attacked, all the others must come to its defense.

On the campaign trail, Donald Trump said he’d be reluctant to defend countries that hadn’t paid their fair share toward collective defense, and just last month Mr. Trump’s new Secretary of Defense James Mattis visited NATO Headquarters with what can only be described as a mixed message.

“It’s fair to demand that all who benefit from the best defense in the world carry their proportionate share of the necessary cost to defend freedom,” he said.

A U.S. tank takes part in an Independence Day parade in the Estonian capital of Tallinn, in early March, 2017. CBS

NATO guidelines say members must spend two percent of their GDP on defense. Right now only five of the 28 NATO member countries do that, including the U.S.

When Secretary Mattis warned that America might moderate its support of the alliance if the other NATO allies fail to increase their spending, what did he mean?

“He meant that is extremely important that the European allies invest more in defense,” Stoltenberg said. “Just a few of them are now meeting the guideline of spending two percent of GDP on defense.”

But how did the NATO chief interpret Mattis’ suggestion of a theoretically “moderated” U.S. stance, should those commitments not be met?

“The way I understand it is that, of course there is a limit for how long the United States is willing to guarantee the security of Europe if not the Europeans are also stepping up. That’s a clear message and I think that everyone here in Brussels and in Europe understood that message,” Stoltenberg said.

The secretary general appeared to welcome the plea for unity, but also wanted to remind U.S. officials that the alliance has helped maintain stability on both sides of the Atlantic for 70 years -- it is to the benefit of all members, not just the European ones, in other words.

“I think that it is useful that the United States so clearly conveys what they expect, but at the same time this is not just about the U.S. asking Europe to do more. This is about 28 allies from Europe and North America sitting around the same table in September 2014 and they agreed together to invest more in defense. Two world wars and the Cold War have taught us all that stability in Europe is also important for the United States.”

Secretary General Stoltenberg stressed that he has no problem with Washington’s message., but what about Moscow?

There is a danger that this U.S. pressure on NATO members to pay more -- or else -- could be viewed by Vladimir Putin’s Kremlin as a sign of cracks appearing in the alliance that has held Western democracies together since World War II.