As tensions increase in several geopolitical hotspots, "the U.S. has to continue to exert leadership," according to former U.S. Secretary of Defense.
William Cohen—a former Republican Senator from Maine who served in President Bill Clinton's cabinet from 1997 to 2001—said he would have hoped President Barack Obama had traveled to Europe to attend funeral services for victims of Malaysia Flight 17.
Such a trip would have demonstrated "shared sentiment and sorrow," as well as afford the president the opportunity to participate in meetings in Brussels with European Union and NATO leaders, Cohen said.
Cohen cited international intelligence suggesting that Russian President Vladimir Putin has been supplying weapons to separatist groups in eastern Ukraine. The European Union's executive European Commission is considering imposing sanctions on state-owned banks in Russia, thereby cutting off access to capital markets across the continent.
"I think it's a real missed opportunity on the part of the United States not to try to rally our European friends in a way that would send a signal to President Putin that this has got to stop."
—By CNBC's Katie Kramer