It isn’t countries doing this. It is companies. Just because a company has access to a resource, and someone in the same country wants that resource, it doesn’t mean they have arranged to sell to the closer buyer. They may have a better deal to sell to someone in another country. The same goes for the buyer. They might have a better seller lined up elsewhere.

It sounds inefficient, but not necessarily, depending on how the supply lines are set up.

It's often superficial similarities that get called out. Like crude oil. People talk about crude oil like it's all the same, when in fact it's very different based on where it's coming from: Not all countries have the capability to refine all oil.

So, it might make sense to import oil from a source that your country can refine and export your domestic-derived oil to other countries that can refine it, perhaps at a higher price or for geo-political reasons.

There can be a lot of reasons. First, just because something is in the same country doesn't mean it's close. Conceivably, people in western Canada might import a resource from Washington State, and people in Maine might import the same resource from eastern Canada. So it looks like Canada and the US are just swapping resources pointlessly, but it's actually easier and more convenient for everyone to do it that way. Another possibility is that the same resources might be available at different times of year. Maybe the strawberry crop comes in from May to July in Mexico, and from July to September in the US. So in the earlier part of the year, Americans import Mexican strawberries, and later in the year, Mexicans might import American strawberries. Another possibility is that there are differences in the resources available in the respective countries. Americans import Swiss chocolate, which is a fancy luxury good. Swiss people may import American chocolate, which is a cheaper, tasty bad e market candy. If you just see a spreadsheet that is labeled "chocolate", it sounds pointless, but it's really fairly different stuff.

Sometimes the country made a deal to export a certain quantity of something by a certain date, but in the meantime domestic demand rose, so they have to fill the deal with exports, and import to cover what the locals are buying.

Sometimes, through a series of financial jiggery-pokery well beyond ELI5, it's profitable. Other times, there are differences that just don't show up in general statistics.

For example, the US both imports and exports crude oil. This looks crazy. BUT... the US refining industry is set up to refine the specific kind of oil they import, which is low in sulfur, and exports the local high-sulfur oil to places where they have a refining industry equipped to deal with that already in place. But why, I hear you ask, not refine local oil locally? It's more cost-effective to swap oils around than rebuild refineries.

Or another country might be importing and exporting "alcohol", which also looks crazy... but if one is high-quality alcoholic beverages and the other industrial-grade solvents... not so much.

It happens when intranational transportation costs more than international for that resource/location combination

Many nations have minimal tariffs on trade from their neighbors.

For the US steel is something that freely crosses borders. Toronto may import steel from Pittsburgh while Minnesota imports from just across the Canadian border because there wasn't a price premium on it. The end result is that the US both important and exports steel but to different locations

It's all about if getting things internally is cheaper than externally. If that changes across the length of the country then some parts may import while others export

International trade is complicated.

Say we have 3 countries;

Country A

• has companies that produce steel
• has a manufacturing base that consumes steel
• has trade agreements with country B, and country C on steel trade providing lower than normal tarrifs

Country B

• has companies that consume steel

Country C

• has companies that produce steel

It's possible that country A's steel producers can make easier, or even higher profits selling steel in bulk to country B than if they were to service their manufacturing base.

At the same time, their manufacturing base could get cheaper steel from country C if, for example, it costs them less to make it.

Companies in country B, can't trade with companies in country C at the same cheaper rate, because the countries don't have a trade agreement setting lower costs on the steel trade for whatever reason.

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Countries are not individuals. A country isn't a single unified entity.

There are lots of people and groups and corporations inside a country that may act independently from each other.

A company deciding to import something while a different company in the same country exports the same thing may make sense for them both, especially when the trade barriers are low.

Countries also aren't always small. Importing something in one side of a country while exporting it on the other side might make sense when that is easier and more profitable than trying to meet an internal demand by transporting the resource across the country.

There are also other factors like timing when the a few weeks after something was exported importing the same resource becomes a good idea again.

People and companies also may have contracts and commitments they made before that make them import and export stuff when it would not make sense for others in the same place and time.

All this mostly applies to free market economies. In planned economies where there is in theory one central coordinated body making all decisions many of these factors would be lessened. In practice it still happened though.

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This is actually a continued subject of debate/study among economists, but the leading theory right now is that it is driven by a taste for variety. In essence, a country may only be able to support three or four major car manufacturers, but there are millions of people in that country that all like different kinds of cars. The best way to continue to capitalize on economies of scale and still deliver on the widest variety of cars for your consumers is to trade between countries/manufacturers.