The Hill | LAWRENCE J. HAAS: In Latin America, a U.S. retreat that began under President Barack Obama has accelerated under President Donald Trump, creating a vacuum that China, Russia, and Iran are moving to fill.

The most fruitful opportunity for those U.S. adversaries lies in the socialism-ravaged state of Venezuela, where each of them is jockeying for position, but opportunities abound elsewhere as well.

It�s just one more example � though a particularly noteworthy one, coming in America�s own backyard � of how the U.S. withdrawal from its post-war global leadership is creating opportunities for authoritarian powers with anti-American designs to expand their influence far and wide.

�The era of the Monroe Doctrine is over,� Obama�s Secretary of State, John Kerry, declared in late 2013, renouncing a doctrine of 190 years through which the United States made clear that it would not tolerate interference by outside powers in the Western Hemisphere.

Kerry�s words reflected Obama�s refusal to address outside inference in Latin America, by (1) Iran, as it nourished economic and military ties to the anti-American �Bolivarian alliance� of Venezuela, Bolivia, Ecuador, and Nicaragua and (2) Hezbollah, Iran�s key client, as it conducted activities from its regional headquarters at the crossroads of Paraguay, Argentina, and Brazil.

It was a far cry from the spirit of President Kennedy�s inaugural address in which, after promoting his Alliance for Progress for the region, he renewed the Monroe Doctrine with this clarion call: �[L]et every other power know that this hemisphere intends to remain the master of its own house.�

While Obama left a largely unchallenged playing field south of the U.S. border for Iran and Hezbollah, Trump is creating an even larger one for not just Iran and Hezbollah but, more ominously, China and Russia.

In Venezuela, Trump called for the ouster of the thuggish socialist President Nicolas Maduro and put Washington behind opposition leader Juan Guaido, whom it and much of the West considers the nation�s legitimate President.

To help push Maduro overboard, Trump slapped oil sanctions on Venezuela, which deepened its severe economic crisis and exacerbated the suffering of its people but haven�t forced Maduro out. Nor, despite U.S. hopes, has Maduro�s military, his most important source of domestic support, turned on him.

With, as critics have noted, no �Plan B� beyond sanctions, Washington now lacks influence in Caracas to persuade Maduro to change course. Not surprisingly, the besieged Maduro has sought support elsewhere, and China, Russia, and Iran have stepped up with efforts to strengthen him and weaken the United States.

Both Russia and China have invested billions of dollars in Venezuela. For China, it�s part of a five-year pledge for Latin America and the Caribbean of $250 billion in direct investment and $500 billion in trade. Meanwhile, Russia recently sent 100 troops to Caracas that include both special forces and cybersecurity experts who, together, can protect Maduro and perhaps spy on his opposition.

Beyond attacking Maduro, Trump�s hostile approach to a host of countries for a host of reasons is pushing them away, creating additional opportunities for China and Russia to make regional inroads.

As the Washington Post summed over the weekend, Trump imposed sanctions on Cuba and Nicaragua for supporting Maduro, recalled diplomats from Central American countries who don�t recognize Taiwan, slapped steel tariffs on Brazil, and said he might send U.S. troops to Mexico to battle cartels.

Meanwhile, Trump announced plans to cut hundreds of millions of dollars in aid to Honduras, El Salvador, and Guatemala in what could be a self-defeating effort to curb immigration. Though Trump blames those nations for not curbing the flow of immigrants to America�s border, the funds that he�s cutting were designed to address such root causes of immigration as violence, poverty, and unemployment.

More striking, and more harmful to America�s long-term regional relations, Trump�s decision of late March came just a day after Homeland Security Secretary Kirstjen Nielson � whom Trump unceremoniously dumped about a week later � signed a Memorandum of Cooperation (MOC) in Tegucigalpa with security ministers of the same three countries that are losing U.S. aid, and she thanked them for their �continued collaboration and partnership� to �stem the flow of irregular migration.�

Trump�s announcement of the aid cuts surely caused heads to spin in Tegucigalpa, San Salvador, and Guatemala City, which invested years in the efforts that culminated in last month�s MOC with Washington.

More importantly, the U.S. regional retreat of recent years is pushing Latin nations into the arms of U.S. adversaries who probably wonder how they got so lucky.

Lawrence J. Haas, senior fellow at the American Foreign Policy Council, is the author of, most recently, "Harry and Arthur: Truman, Vandenberg, and the Partnership That Created the Free World."