Donald Trump publically stated that he would not defend NATO countries that did not pay their contributions in the event of a Russian invasion and would even encourage Moscow's attack.
According to NATO Secretary General Jens Stoltenberg, this comment has grave implications and could put American and European soldiers in danger.
Trump has shown an apparent lack of commitment to the mutual defense of Alliance member countries and has aimed to reduce US contributions, forcing each member country to boost their own armies.
Trump's proposal could spur Moscow's expansionary interference, benefit China and Iran, and ultimately destabilize the international order.
Trump's understanding of NATO is incorrectly; Atlantic Treaty members do not pay the US, but contribute to a common fund.
Conclusion: If Trump returns to presidency and pursues an isolationist policy, it could cause a destabilization of the international order and force Europe to fend for itself against possible threats.