In article in Britain's Observer newspaper, days after Trump's US election win, Secretary General Stoltenberg said, the West faced its greatest security challenge in a generation.
During his election campaign, Trump described Western military alliance NATO as obsolete.
He suggested that the US would think twice about coming to the aid of any NATO ally under attack if it had not paid its dues.
"We face the greatest challenges to our security in a generation," Stoltenberg wrote while conceding that Trump had a point about the need for some members to make a bigger financial contribution, as the US currently accounted for almost 70 per cent of NATO spending.
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"It is all too easy to take the freedoms, security and prosperity we enjoy for granted. In these uncertain times we need strong American leadership, and we need Europeans to shoulder their fair share of the burden," the former Norwegian prime minister wrote.
"Going it alone is not an option, either for Europe or for the United States. We face the greatest challenges to our security in a generation. This is no time to question the value of the partnership between Europe and the United States."
"This was more than just a symbol. NATO went on to take charge of the operation in Afghanistan. Hundreds of thousands of European soldiers have served in Afghanistan since.
"And more than 1,000 have paid the ultimate price in an operation that is a direct response to an attack against the United States.
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