• The U.S. has used tariffs since its founding in the 18th century.
• They were primarily a way to raise revenue in the nation's early days. Later, tariffs were largely used to restrict imports or as a bargaining chip to reduce trade barriers.
• President Donald Trump's use of the import duties has broken with historical norms, economists and historians said.

President Donald Trump broad tariffs on China that took effect Tuesday, while his tariff threats other major trading partners: Canada, the European Union and Mexico.

That may lead some to wonder: How have tariffs been wielded throughout U.S. history, and is Trump's use of them unusual?

The 'three Rs' of tariffs

The U.S. has used tariffs since its founding in the 18th century.

In fact, the Tariff Act of 1789 was among the first bills ever passed by Congress.

Since then, the U.S. has used tariffs to achieve three broad goals, said Douglas Irwin, an economics professor at Dartmouth College and past president of the Economic History Association.

Irwin calls them the "three Rs": revenue; restriction, or import barriers to protect domestic industry; and reciprocity, a bargaining chip to cut deals with other countries.

Using tariffs for revenue

Tariffs are taxes on U.S. imports, paid by the entity that's importing the foreign goods. Those taxes raise revenue to help fund the federal government.

For roughly the first third of the nation's history — from its founding until the Civil War — the revenue motivation was "paramount" as a driver to impose import duties, Irwin said. The federal government relied on tariffs for about 90% or more of its revenue during that period, he said.

But things changed after the Civil War, Irwin said. The U.S. started to impose other taxes, such as , that made the nation less reliant on tariffs.

Tariffs generated of federal revenue from about 1860 to 1913, when the income tax was created, Irwin said.

The scale of the government expanded significantly in the 1930s — with the creation of New Deal programs such as Social Security — and later for defense spending during World War II and the Cold War, said Kris James Mitchener, an economics professor at Santa Clara University who studies economic history and political economy.

Today, "tariffs simply to fund government expenditure," Mitchener said. "There's no possible way you could support the size of the U.S. military on tariff revenue."

Restriction and reciprocity

From the Civil War to the Great Depression, the U.S. primarily used tariffs as a restrictive measure on imports, to insulate the domestic market from foreign competition, Irwin said.

The Tariff Act of 1930, popularly known as the Smoot-Hawley Tariff, on about 800 to 900 different types of goods, accounting for about 25% of all goods imported to the U.S., Mitchener said.

The U.S. also used tariffs as a reciprocal bargaining chip.

For example, before the U.S. annexed Hawaii, it a free-trade agreement with the Kingdom of Hawaii in 1875. The treaty allowed for duty-free imports of Hawaiian sugar and other agricultural products into the U.S. In exchange, the U.S. to the harbor that would later be known as Pearl Harbor.

The post-Depression, and especially the post-World War II period, became an era of reciprocity, Irwin said.

The U.S. helped create the in 1948, the precursor to the World Trade Organization, which set global rules for trade and ushered in an era of low tariffs.

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How the president's tariff power grew

U.S. import taxes before the WWII era were pretty high, ranging from 20% to 50%, sometimes even reaching 60%, Irwin said. They have been "very low" since 1950 or so, he said.

The average duty on goods subject to a tariff was about 2% to 4% in the 2010s before Trump's first term, Mitchener said.

"That's what President Trump is trying to overturn, this sort of low period of tariffs we've had since World War II," Irwin said.

Before 1934, Congress — not presidents — had power over tariff rates and negotiations, said Andrew Wender Cohen, a history professor at Syracuse University.

But Democrats — then known as the political party of free trade — had an enormous majority around the New Deal era and passed the , granting the president the right to negotiate tariffs in certain cases, Cohen said.

"That's when the president gains a much more substantial authority," Cohen said.

That power accelerated after 1948 during the "transformation of the whole global economic order," he said.

Why Trump tariff policy is 'very unusual,' economists say

Trump's use of tariff policy is "very unusual" among modern U.S. presidents, Cohen said.

For one, Trump "likes all three Rs" — revenue, restriction and reciprocity, Irwin said.

On the campaign trail, Trump suggested that tariffs to fund the government. He during his campaign that tariffs would create U.S. factory jobs and has to strong-arm Denmark to give up Greenland.

However, there are trade-offs, Irwin said. Restricting imports somewhat negates tariffs' ability to raise revenue, because it diminishes the tax base for tariffs, he said. Those additional duties may cause companies to import fewer goods or may push people to buy less, for instance.

"You can't really achieve all three objectives at the same time," he said.

Additionally, no previous president has tried to link a U.S. drug crisis to trade policy, as Trump did with fentanyl.

"That's a novel take," Mitchener said.

Many presidents have used tariffs. For example, George W. Bush, Ronald Reagan and Richard Nixon applied tariffs to protect the U.S. steel industry, as Trump , Irwin said.

"What's unusual about Trump is, he's not just picking out particular industries that he thinks are of strategic importance, but he's blocking imports across the board almost with some of these countries," Irwin said.  

Trump imposed a 10% additional tariff on all Chinese goods and threatened a 25% tariff on imports from Canada and Mexico.

"No president in recent memory has really used tariffs across the board or in a broad-brush way to achieve various objectives," Irwin said. "They've sort of adhered to the rule that we belong to the WTO. That means we keep our tariffs low as long as other countries keep their tariffs low."

Global trade treaties, such as the United States-Mexico-Canada Agreement, which Trump signed in his first term, establish a mechanism for nations to file grievances for alleged unfair trade practices, Cohen said. Nations can generally raise tariffs as a retaliatory measure if trade rules are breached, according to the treaty terms, he said.

Trump's recent unilateral tariff announcements are unique in this regard, he said.

"I can't think of any precedent for that," Cohen said.

"While the executive branch was given much more power since 1934, it's always been subject to the specific terms of the agreements," he said.