President Trump’s abbreviated Hanoi meeting with Kim Jong Un reveals again the limits of personal diplomacy. After 2017’s “fire and fury,” Trump in early 2018 abruptly welcomed a summit with Kim. Trump basked in Singapore’s photo-ops but left with nothing solid.

Trump ignores analysts and diplomats; in his real-estate world, everything is personality-driven. Recently in Hanoi, Trump grandly offered to end sanctions if North Korea abandons all nuclear weapons. Kim offered inspections of just one site (Yongbyon) in exchange for lifting the painful U.S. sanctions. Both left Hanoi with nothing.

We overfocus on denuclearization, which ain’t gonna happen. Pyongyang will not give up its nukes but knows that using them would turn the peninsula into an insula. Unavoidable result: a U.S.-North Korea nuclear-deterrence standoff.

Bigger picture: It ain’t the nukes; it’s the economy. North Korea, after years of famine, is quietly allowing a partially market economy, and life is improving. But sanctions stall growth. Easing sanctions would encourage markets and trade, both big threats to nightmare-regimes. South Korea, eager for peace, leads here.

North Korea is following China’s earlier path, but this fosters insoluble contradictions and dilemmas. When a Communist dictatorship decentralizes and liberalizes its controlled economy, it grows — in the case of China, spectacularly. Growth brings freedom, autonomous power centers and massive corruption. Individuals seek profits, found enterprises, and, especially with foreign investment, produce for the world market. Pluralism sprouts; communism fades.

The regime fears this and cracks down, but that chokes off economic growth, so after a few years of central control it opens up again. Economic policy never finds stability; it zig-zags between tighter and looser as the regime flits between control and prosperity. Communists suppose they can have both, but not even China has found a stable amalgam.

I first realized this when I studied in Belgrade in 1963-64, intrigued by Yugoslavia’s “third way” between capitalism and socialism. Back then, many discussed a “convergence” of Western market systems and Soviet communism. The two models could meet in a middle ground. Even President Eisenhower mentioned it.

The Yugoslav third way looked good from the outside but was actually a mafia-type operation. Every few years, Tito would decide that either tightening or loosening had gone too far and reverse course. No shift was his last because the problem was never solved. On top of underlying economic dysfunction, the departure of their constituent republics, both in 1991, dissolved both Yugoslavia and the Soviet Union. Yugoslavia suffered hideous ethnic cleansing.

In the Soviet Union in the early 1960s, Khrushchev permitted discussion of adopting certain capitalist tools such as flexible prices and charging interest. This vanished with Khrushchev’s 1964 ouster, but a generation of young Soviets got to play with notions of economic liberalization. One of them, Gorbachev, later tried to implement a clumsy middle ground. Its failure doomed the Soviet Union.

China has fallen into the zig-zag. In December 1978, Deng Xiaoping announced a move to markets. It began in 1979 and, accelerated by Deng’s 1992 Southern Tour, allowed new private sectors in China’s economy, with record-setting results. Banks and state-owned enterprises remained in regime hands.

When Xi Jinping took power in late 2012, he worried (correctly) that continuing on the market path eroded the Party’s monopoly on power. So he reversed course, and both China’s nascent freedoms and economic miracle constricted. Xi favors state-owned enterprises, many of them money-eating zombies. China’s growth has slowed, probably below 2018’s official 6.6 percent. Xi alternatively loosens and tightens bank credit, always overcorrecting for the previous phase. Worry mounts. Better-off Chinese try to get their money and themselves out of China.

Communist regimes are under pressure to partially marketize because they see they are falling behind. Capitalist economies, buoyed by competition and venture capital, perpetually innovate. Communist economies get stuck copying old technology. Example: From 1947 to 1986, the best Soviet camera was a copy of the 1936 German Zeiss Contax whose machine tools were shipped to Kiev as war reparations. (China is trying to innovate by massive investments in R&D. Let’s see how they do.)

Eventually, Communist regimes realize that control and growth are contradictory. Kim Jong Un’s inner circle surely discusses this dilemma. Four U.S. presidents have offered Pyongyang economic incentives, but it pulls back, fearful of change that undermines the system. They’re right.

Trump’s extrapolation of business bravado onto the world scene doesn’t work, but time may. North Korean entrepreneurs already operate by bribing authorities (same happened in China), forming a small middle class. Pyongyang will not end corruption any more than China has. Anti-corruption drives are like mowing weeds; they grow right back. Draconic penalties do not solve problems embedded in the economy. Ironically, Marx wrote at length on economic contradictions but called them a capitalist problem. Mr. Kim, welcome to the zig-zag.