Foreign corporations are the ones that stole Venezuela's oil

During a press conference to announce the U.S. invasion of Venezuela and the kidnapping of President Maduro, Trump said that the country had seized and stolen "American oil, American assets, and American platforms, costing us billions and billions of dollars."  This echos things he's said in the past, demonstrating that the invasion had nothing to do with narco-trafficking and everything to do with taking over the world's largest known oil reserves.

As a country, we must now face the fact that, during the Cold War, U.S. proxy battles all over the globe (but especially in Central and South America) were framed as "good U.S. vs. bad USSR." As a result, many people are ignorant of the actual history.

When it comes to Venezuela, significant oil discoveries began in the 1920s. By the 1930s right-wing strongman Juan Vicente Gómez granted concessions (including generous kickbacks to himself and his cronies) that left three foreign oil companies - Shell, Gulf (Chevron), and Standard Oil - in control of 98% of the Venezuelan market. In other words, three companies (two of them American), seized 98% of Venezuela oil. 

By the 1940s, Venezuela began attempts to claw back these oil profits. For example, under President Isaías Medina Angarita, authorities approved a law in 1943 that required foreign oil companies to relinquish half their profits to the government.

Then, in 1975, President Carlos Andrés Pérez, a democratic socialist, signed a bill that nationalized Venezuela's oil industry. Here's what the New York Times reported a year later:

A year after nationalization of Venezuela's $5 billion petroleum industry, the foreign concerns that built and controlled it—principally ExXon, Royal Dutch/Shell, Mobil and Gulf—are receiving approximately $1 billion in compensation and continue to provide marketing outlets and technological aid under contract.

Although the industry accounted for one of the world's largest pools of foreign investment, the takeover, in contrast to the abrupt Chilean nationalization of American copper companies in the early 1970's, has been peaceful and orderly, following months of negotiations between the Government and the 22 foreign concessionaries.

These kinds of changes were happening all over the globe. 

[Venezuela] followed Mexico, Brazil and Saudi Arabia in a wave of resource nationalism aimed at trying to wrest control of energy resources, primarily from the United States, to achieve economic sovereignty.

Since the 1970s, Venezuela has had times when they extended partnerships with foreign oil companies and times when they pulled back. It is not necessary to support or condemn the actions of any one leader to understand that the critical change Trump is referring to happened 50 years ago when Venezuela took back what foreign oil companies had stolen from them.