Washington (CNN) NATO Secretary General Jens Stoltenberg delivered a pointed and robust defense of the trans-Atlantic alliance to Congress on Wednesday, stressing the need for unity in the face of deep global shifts and "unforeseen" challenges ahead.

Stoltenberg indirectly addressed President Donald Trump's criticism of NATO and his push for an "America First" unilateral approach to the world throughout the speech and touched on trade and economic relations, another point of tension between the Trump administration and Europe.

"When we stand together, we are stronger than any potential challenger -- economically, politically and militarily," said Stoltenberg, the first NATO chief to address a joint meeting of Congress.

"We need this collective strength, because we will face new threats," Stoltenberg said. "We need a strategy to deal with uncertainty. We have one. That strategy is NATO."

"Questions are being asked on both sides of the Atlantic about the strength of our partnership. And yes, there are differences," the NATO chief acknowledged, listing climate change, trade and the Iran nuclear deal. "Open discussion and different views is not a sign of weakness. It is a sign of strength."

'Most difficult problem'

Stoltenberg delivered his remarks to mark NATO's 70-year history as the trans-Atlantic alliance faces unprecedented challenges, chief among them, according to veteran US and foreign diplomats, Trump and "the absence of strong, principled American presidential leadership."

"Trump is regarded widely in NATO capitals as the Alliance's most urgent, and often most difficult, problem," said the February report by Harvard's Belfer Center for Science and International Affairs and its Kennedy School based on interviews with current European and North American leaders, former senior officials, academics and journalists.

The report notes "Trump's open ambivalence about NATO's value to the US, his public questioning of America's Article 5 commitment" which mandates that members come to the defense of any nation in the alliance that's been attacked, as well as his "persistent criticism of Europe's democratic leaders and embrace of its anti-democratic members and continued weakness in failing to confront NATO's primary adversary President Vladimir Putin of Russia."

All these factors, wrote the Harvard study's authors, Nicholas Burns, a former Undersecretary of State appointed by President George W. Bush, and Douglas Lute, a former US ambassador to NATO from 2013 to 2017, "have hurtled the Alliance into its most worrisome crisis in memory."

The NATO chief threaded his 39-minute address, which was repeatedly interrupted by applause, with references to shared sacrifices of blood and treasure -- particularly in NATO's continued deployment to Afghanistan since the terror attacks of September 11, 2001. Europe and Canada have also provided the US with a platform to project power, access to intelligence and cyber capabilities and partnerships on next-generation military capabilities that create jobs in all three places.

Stoltenberg noted that while there have been moments of deep disagreement within the alliance before -- he cited the US invasion of Iraq in 2003 -- "the strength of our alliance is that we've been able to unite around keeping our people safe."

'Obsolete'

"We have overcome our disagreements in the past and we must overcome our differences now, because we will need our Alliance even more in the future," Stoltenberg said. "Our Alliance has not lasted for 70 years out of a sense of nostalgia, or of sentiment," he said. "NATO lasts because it is in the national interest of each and every one of our countries."

The alliance faces challenges that include a shifting balance of global power, an assertive Russia, cyberthreats, the wild card of technologies such as artificial intelligence and quantum computing, the conflict in Afghanistan and terrorism, Stoltenberg said. He didn't list the challenge posed by alliance members such as Turkey and Italy , which are allying themselves more closely with Russia and China.

The 29-member group has also had to contend with Trump's repeated criticisms, among them his declaration that the alliance is "obsolete" and his frequent, public expressions of frustration that most NATO allies do not meet the two percent of GDP target that the alliance recommends members spend on defense.

The President's supporters that say allies should not focus on everything Trump says, but on what his administration does. "Some leaders may be dissatisfied with President Trump's rhetoric," said Nile Gardiner, a director at the Heritage Foundation, "but to assert that he is not committed to the NATO alliance and our partners in Europe does not match up with the reality of US policy."

Gardiner points to the Trump administration increasing troop presence in Eastern Europe, investing more in the European Deterrence Initiative, a rapid reaction force against potential Russian aggression, its consideration of a US troop presence in Poland and its sanctions against Russia. And he flags the President's insistence that NATO members spend more on their defense.

Stoltenberg, in his only mention of the President, praised Trump for the "clear message" that the alliance must boost spending, adding that "this message is having a real impact."

'This is making NATO stronger'

"In just the last two years, European Allies and Canada have spent an additional 41 billion dollars on defense," Stoltenberg said. "By the end of next year, that figure will rise to one hundred billion. This is making NATO stronger."

Asked about the Harvard study at a Monday press conference in Brussels, the NATO chief said, "sometimes I agree, sometimes I disagree, but the fact is that we see that North America and Europe are doing more together than they've done for many, many years."

Stoltenberg pointed to reasons the alliance should continue to work together in the years to come, including Afghanistan and the fight against terror groups such as ISIS, but he gave Russia special emphasis.

Moscow's increasing aggression includes disinformation campaigns and electoral interference, cyber attacks, support for the Syrian regime, interference in Ukraine and Crimea's annexation, the use of a military-grade nerve agent in the US, as well as "a massive military build up from the Arctic to the Mediterranean and from the Black Sea to the Baltic."

US President Donald Trump (L) walks with NATO Secretary General Jens Stoltenberg as he arrives to attend the NATO (North Atlantic Treaty Organization) summit, in Brussels, on July 11, 2018. (TATYANA ZENKOVICH/AFP/Getty Images)

Stoltenberg pointed in particular to Russia's deployment of mobile, nuclear capable missiles in Europe that he said are "hard to detect. ... cut the warning time to just minutes and reduce the threshold for the use of nuclear weapons in an armed conflict."

"I continue to call on Russia to return to compliance with the INF Treaty, but so far, Russia has taken no steps to do so," Stoltenberg said. "And time is running out."

"NATO has no intention of deploying land-based nuclear missiles in Europe," he said, "but NATO will always take the necessary steps to provide credible and effective deterrence."