President Donald Trump is showing America’s trading partners no mercy. If they export a lot to the U.S., it will tariff them back — by half. The rates are especially steep on many developing countries, with Vietnam, India, Thailand and Bangladesh all well over 20%.

The costs won’t be felt equally. Tariffs aren’t great for the U.S., but trade is still a relatively small part of its economy (less than 25%), and if there is a corporate tax cut, the costs to U.S. consumers may not be as high as feared. But such high tariffs will be catastrophic for developing countries that depend on exports. The costs could be far worse than cutting foreign aid, including eliminating USAID.

Foreign aid does not have a great track record. Aside from its potential for corruption, it can distort economic decisions and actually set back economic development. If the goal is increasing growth and reducing poverty, export-oriented growth has historically been more successful. Opening up to trade encourages economic activity, increases inflows of foreign capital and promotes more investment and sustainable growth. The success of the so-called Asian Tiger economies, for example, was largely because of export-driven growth.

If their exports to the U.S. now face higher prices, developing countries could face a deep depression, civil unrest and more severe poverty. It may not be in America’s interest, either. Tariffs will not cause the U.S. to become a center of low-skill manufacturing. And if their purpose is to force U.S. companies to move their supply chains from China for national security reasons, why are the best alternatives — such as Sri Lanka, Cambodia and Vietnam — now facing even higher tariffs?

It is possible, of course, that this tariff policy won’t be a disaster. Under the best-case scenario, the countries slapped with high tariffs would respond by cutting their own tariffs on imports.

This would be difficult and controversial. It has been argued that the East Asian miracle happened in part because these countries used tariffs and subsidies to give their industries a chance to grow and become internationally competitive. This is one reason that many developing countries have high tariffs today.

In most countries, however, protectionism as a means to economic development has a mixed track record. It often does more harm than good because, like aid, it can distort economic decision-making, stifle competition and foster corruption. In general, freer trade is a better way to grow. The Indian economy, for example, could become the world’s largest if it had fewer trade restrictions.

But governments don’t always act rationally, present country included. And even if developing nations cut their tariffs, they would still face the U.S.’s minimum 10% tariff rate. The bottom line is that tariffs risk a global trade war with developed markets, which would be economically disastrous and erase much of the dramatic decline in worldwide poverty that was the great triumph of the late, lamented neoliberal era.

Allison Schrager is a Bloomberg Opinion columnist covering economics. A senior fellow at the Manhattan Institute, she is author of “An Economist Walks Into a Brothel: And Other Unexpected Places to Understand Risk.” This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.