President Trump has said repeatedly that Nato is obsolete. And he is right. For five decades Nato was necessary to, as its first Secretary General said at the time, “keep the Soviets out, the Americans in, and the Germans down”.
The Atlantic Alliance was indeed instrumental in deterring the USSR and keeping the European continent in peace. There were many discussions as to how to achieve its goals, including major disagreements, but with America providing leadership, taking a big portion of the economic burden, and being willing to station hundreds of thousands of GIs in Europe, the Allies were able to overcome all problems and stay highly successful during the Cold War.