WASHINGTON — Three nations that have long defined themselves as bitter adversaries of the United States — North Korea, Iran and Venezuela — decided this week they could take on President Trump.

Each one is betting that Mr. Trump is neither as savvy a negotiator nor as ready to use military force as he claims. Each also poses a drastically different challenge to a president who has little experience in handling international crises, has struggled to find the right balance of diplomacy and coercion and has not always been consistent in defining his foreign policy.

The rising tensions with all three serve as reminders that Mr. Trump’s constant talk about taking care of problems that he has accused his predecessors of aggravating, or failing to confront, is difficult to convert into real-world solutions — as events of recent days have shown.

The confrontation with Iran appears to be the most volatile at the moment, with tensions escalating by the day. On Friday, the Pentagon said it was sending another naval ship and Patriot missile interceptor battery to the Middle East, in addition to an earlier dispatch of a carrier group and bombers, because of potential threats from Iran or allied Arab militias.