Fillon, a major contender in France's two-round presidential election on April 23 and May 7, proposed to put in place a European defense alliance led by France and Germany.
"The Islamic totalitarian threat ... Along with the weakening of (NATO) through President Trump's rhetoric and contradictory signals, are forcing us to overhaul our alliances and take a second look at the question of European defense, which was neglected for too long," he told a news conference.
"For decades, we have lived with the illusion of perpetual peace, the irreversible nature of the European Union and the unconditional commitment of the U.S. Toward the safety of Europe. These times are over," he stressed.
Fillon also took a harder stance on Russia, contrasting with earlier comments calling for warmer relations.
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"Its political regime is not identical to the ones in our Western democracies. It is a dangerous country," he said.
This strategy consists of "a serious and honest dialogue to bring about the conditions of Europe's safety" rather than to confront Russia, he said.
Fillon, once considered the presidential front-runner, has dropped in the polls following revelations he employed family members for years for parliamentary jobs they allegedly never performed. He is facing charges in the case. He denies any wrongdoing.
Far-right leader Marine Le Pen and independent centrist Emmanuel Macron are now considered the two top contenders in the presidential race. The two top vote-getters on April 23 compete in the presidential runoff on May 7.
"We cannot be member of NATO, because it's heading on a war path on the European continent," he told reporters on today.
Melenchon also opposes the idea of a European defense alliance proposed by competitors like Fillon and Macron "because European defense, as it is currently formulated, it's war... And to be more precise, it's war with the Russians. And this is absurd.