The top U.S. military officer is warning that the war in Ukraine will likely last years, raising concerns that the world "is becoming more unstable and the potential for significant international conflict between great powers is increasing, not decreasing."

During testimony in front of the House Armed Services Committee on Tuesday, Chairman of the Joint Chiefs of Staff General Mark Milley called Russia's invasion of Ukraine "the greatest threat to peace and security of Europe, and perhaps the world, in (his) 42 years of service in uniform."

"The United States is at a very critical and historic geostrategic inflection point. We need to pursue a clear-eyed strategy, maintaining the peace with the unambiguous capability of strength relative to China or Russia," he said, referring to the top two potential military threats to the U.S.

Asked by lawmakers what could have stopped Russian President Vladimir Putin from attacking Ukraine, Milley said the only defense possible may have been to put U.S. forces inside the country, which he would not advise because it would have risked an armed conflict with Russia.