• President-elect Donald Trump's election win last week has fueled fears of an impending economic nightmare for the European Union.
• François Hollande, the former socialist leader of France, led the nation between 2012 to 2017 and briefly crossed over with Trump's first term.
• He recalled that Trump was often a man of his word.

Former French President François Hollande underlined the importance of Europe sticking together in the face of U.S. protectionism on issues like security and trade.

President-elect Donald Trump's election win last week has fueled fears of an impending economic nightmare for the European Union after he threatened to impose an additional 10% in tariffs on these nations.

This would come at a time when traditional economic powerhouse Germany is dealing with an economic malaise and a political vacuum after its coalition government collapsed. There are also concerns over the Ukraine war and whether the U.S. could pull funding for the war-torn nation — leaving European allies to foot the bill.

Hollande told CNBC's Charlotte Reed Wednesday that Europe must be worried.

"Europe is today a continent that is not united. So what will be absolutely essential is the reaction to what Trump wants to do, particularly the U.S. disengaging from the European continent," he said.

"This is what France, Germany, Poland and the United Kingdom, the four main countries that invest in defense, will do. If these four countries react together, both in relation to the security issue, the Ukraine issue and even the trade issue, then Europe will be respected. Otherwise it will be pushed aside," he added.

Despite Germany and France seeing significant political volatility this year, Hollande said there could still be a "broad political consensus" in these major European nations, no matter who is in charge, that could "firmly resist what Donald Trump may undertake."

Ukraine peace?

Hollande, the former socialist leader of France, led the nation between 2012 to 2017 and briefly crossed over with Trump's first term. He recalled that Trump was often a man of his word.

Trump has previously boasted he could "end the war" in Ukraine in 24 hours if elected, signaling that he would pull the plug on funding in order to force the country into a negotiated settlement with Russia. Analysts say Ukraine could then be pushed into a "bad peace" with its powerful neighbor, and potentially forced to relinquish almost 20% of its territory in the south and east that's currently occupied by Russian forces.

Hollande predicted that Trump will try to propose a peace with Russian President Vladimir Putin that will exclude Ukraine from the negotiations, which could potentially allow Russia to take territories it already occupies.

"Are we going to let it happen? We must not let it happen," Hollande said. "Because it would set an extremely serious precedent where force had prevailed over law."

—CNBC's Silvia Amaro and Holly Ellyatt contributed to this article.