Soldiers hold their national flags during the official welcoming ceremony for NATO's Canadian-led Enhanced Forward Presence combat battalion in Adazi, Latvia, June 19, 2017. REUTERS/Ints Kalnins

NATO shifted its focus to other issues outside of Europe in the years after the Cold War.

But the alliance has in recent years refocused on deterring Russia, which has modernized its military and is more active around the world.

The alliance is now challenged by both conventional threats and irregular tactics, Secretary General Jens Stoltenberg said this month.

New security challenges in Europe require new responses and more vigilance from NATO, according to Secretary General Jens Stoltenberg, who said this month that the lines between conflict and competition are becoming less distinct.

NATO was formed by European and North American countries in 1949 to provide collective defense against Soviet aggression and to help rebuild and integrate Europe. When West Germany joined in 1955, the Soviet Union and several of its satellite states formed the Warsaw Pact.

In the years after the Soviet Union's collapse, NATO's attention shifted to other issues and other regions, but since Russia's annexation of Crimea and incursion in Ukraine in 2014, the alliance has refocused on security and deterrence in Europe.

NATO Secretary General Jens Stoltenberg, front-row middle, and US President Donald Trump with other NATO leaders at the 2018 NATO Summit opening ceremony in Brussels, July 11, 2018. Sean Gallup/Getty Images

Stoltenberg stressed that he did not see "any imminent threat" of conventional military attack "against any NATO ally from Russia or any other adversary."

"But we see Russia, which has modernized its armed forces and used military force against neighbors, Ukraine," Stoltenberg said on a recent edition of the Defense One Radio podcast.

"To make sure that nothing similar to what happened to Ukraine in 2014 could happen to any NATO ally, we have increased our military presence in the eastern part of the alliance," he added, referring to multinational battle groups deployed to Poland and the Baltic states, Estonia, Latvia, and Lithuania.

"We have ... combat-ready battle groups in the eastern part of the alliance, in the Baltic countries and Poland, for the first time in NATO's history to send a very clear message that there is no way that something similar ... to what happened in Ukraine" can happen in those countries, Stoltenberg said, adding that NATO has also increased its presence in Bulgaria, Romania, and the Black Sea.

A soldier from NATO's Enhanced Forward Presence battle group jumps in icy water during the winter-survival exercise in Adazi, Latvia, February 8, 2018. REUTERS/Ints Kalnins

NATO forces have expanded the scope and scale of their exercises in Eastern Europe in recent years, spending more time on operations that had not received as much attention over the past three decades, like maneuvering and logistical support.

Those responses have been about providing credible deterrence to a conventional attacks, but, Stoltenberg said, the alliance isn't only confronted with conventional threats.

"What we see is of course that Russia is in a way ... launching or is responsible for aggressive actions below Article 5, as we call it, below what triggers a full-fledged response from the alliance," he told Defense One.

"That's a challenge, meaning that we need to adapt to a new and more demanding security environment where also there is a more blurred line between peace and war," he added.

Russian President Vladimir Putin delivers a speech during celebrations for Navy Day as it rains in Baltiysk, Kaliningrad region, Russia, July 26, 2015. Reuters

Russian and Russian-backed actors have been accused of targeting NATO personnel in Eastern Europe with hacking and other kinds of information warfare.

Similarly, Russian military officers and intelligence operatives helped coordinate and support the 2014 annexation of Crimea and have stoked the battle against Ukraine in the country's east, a disguised force known as the "green men."

These kinds of irregular tactics are meant to challenge rival forces without triggering a self-defense response, creating "gray-zone" situations were tensions are elevated but don't escalate to open conflict.

China has employed similar tactics in the South China Sea, where it sends its coast guard to enforce its expansive and disputed claims. When the US Navy responds, Beijing has pointed to its presence as US aggression.

During the most recent edition of NATO's Saber Strike exercise, conduced this summer, participants practiced tactics to thwart such interference on land and at sea - including putting their cellphones on airplane mode.

NATO has also said it would boost its own electronic-warfare capabilities. At the end of 2017, the alliance said it would adapt its command structure to integrate cyber weapons - a marked shift from its previous defense-only stance on such operations.

Polish soldiers use the sights of an anti-aircraft cannon to simulate engaging enemy aircraft during a multinational exercise as part of Saber Strike 18 at Bemowo Piskie Training Area, Poland on June 14, 2018. Michigan Army National Guard/Spc. Alan Prince

Such low-intensity tactics often obscure the guilty parties and have created open-ended periods of tension that differ from previous eras, Stoltenberg said.

"The way I grew up was that a war had a specific date when it started and specific date when it ended, and it took place in a well-defined geographic area," he said, citing World War II.

With cyber operations, "it's very hard to tell exactly who attacked you. It's very hard to say exactly where it takes place."

"So we live in a ... completely different security environment with a more blurred line between peace and war, and Russia's efforts to undermine our democratic institutions, to meddle in our political process ... [are] these kind of new threats, new challenges, or part of those new threats and challenges," he added, referring to an attempted coup in NATO's newest member, Montenegro, and the poisoning of former Russian spy Sergei Skripal in the UK, both of which have been attributed to Russia.

"NATO has to be able to respond to that also," he said.

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