The former supreme allied commander of NATO's forces in Europe says the upcoming summit in Poland is the most important meeting of the alliance since the collapse of the Soviet Union.

"This is the biggest summit since the end of the Cold War and I'll tell you why, it's kind of a three-ring circus, three really big issues," retired U.S. admiral James Stavridis said in an interview on CBC News Network's Power & Politics.

"One, of course, is NATO's role against the Islamic State. Another role that is going to be crucial is how NATO steps up in terms of deterrence. And then thirdly it's NATO and its role following Brexit," Stavridis told host Rosemary Barton. "So this is a very big summit for those three reasons principally, and some others as well."

Praising Canadian troops, the retired admiral and dean of the Fletcher School of Law and Diplomacy at Tufts University said there were "no finer" soldiers. He said that "Canada has been a mainstay in the NATO alliance" and would play a key role in the new NATO mission in eastern Europe seeking to deter further Russian aggression.

"We've watched Russia invade Georgia, we've watched Russia invade Ukraine and annex Crimea," Stavridis said. "We want to make sure there is no miscalculation along that border with Russia."

"At the same time, it's crucial that we maintain an open dialogue," he said. "We don't want to stumble backward into a cold war. I think good fences make good neighbours, strong borders make good national neighbours."

The former commander of NATO said Russia has to be convinced that invading other countries is a bad idea, a mission that "unfortunately" could go on for "a long time."

"The prospects of warmer relations with Russia appear very dim for the immediate and perhaps foreseeable future," he added.

NATO and ISIS

Stavridis said he would like to see NATO step up its game against ISIS in Iraq and Syria by increasing the use of special forces, deploying airborne early warning aircraft and establishing a NATO training mission in Baghdad to train that country's soldiers to better battle ISIS.

"NATO has been absent from the strikes against [ISIS]. Some of the individual nations have participated, but as an organization NATO has not stepped up," he said.

To accomplish these and other goals requires a strong NATO, and to that end, Stavridis brushed aside concerns that the recent referendum in which the British chose to leave the European Union would upset the alliance

"Now that the United Kingdom is, evidently, going to pull out of the European Union, those forces will be more available to NATO and actually, perhaps counterintuitively … the weakening of the European Union and the U.K. Brexit, I think, will actually strengthen NATO," he said.

Push for more defence spending

Stavridis said that while all countries make their own internal decisions on how much to spend on their respective defence budgets, Canada should expect a call to spend more when Prime Minister Justin Trudeau meets with other NATO members in Warsaw on Friday and Saturday.

"Neighbours and allies could and should encourage each other to meet goals, and here we have a goal, and the NATO goal, which has been adopted by all 28 nations, is to spend two per cent of GDP on defence," he said.

The former commander said the U.S. spends as much as 3.5 per cent of GDP on defence (Canada spends about one per cent). Other countries should ask themselves how their spending compares to the promises they have made, he said, because reaching two per cent is crucial in today's dangerous world.