Explain Like I'm Five

Tariffs are not meant to be a stimulator of your own economy, they are a more so a test of it's endurance. Let me explain.

Globalism is a double edged sword. We collectively came to the agreement to put it on every country themselves to figure out what they're good at and how they can survive on the global market. It got us as far as we are today. A country can specialize creating a product from the most available resources within it's borders be it natural resources or skilled/unskilled labor. Having access to the global market means way higher profits than just selling to your own people.

The problem with globalism is that it is completely unregulated by an overseeing entity, and since recent times have shown that hostile territorial takeovers are generally frowned upon, every country is essentially stuck with the resources it already got. This means some countries have lucked out and have more resources available than others and are therefore a bigger economic power. Generally the more complex the product your country is exporting is, the higher the state of development your country has. A lot of countries struggle to build a complex industries to meet global demands (see Korea with stem cell reasearch in the 90s and 2000s).

So if territorial takeovers are a no no, then economical takeovers are the peaceful alternative. The problem is that China has the most amount of manpower in the world, it isn't exactly a small country either and therefore has a lot of natural resources. If China was a culturally open and peacefull democracy, this wouldn't be an issue, but as we all know - it isn't. If China decides to take over a market - they can. Additionally the have always been 'rules for thee, not for me', China buys up other countries property and land but doesn't allow others to do the same for it.

Yes, in the end, everybody profits from the efficiency of globalism and open trade but the scales are not evenly tipped for all. This leaves most countries vulnerable to economic attacks from bigger global players. Tariffs are in a way a bargaining chip in the global market. The idea behind it is to say 'I'll take my business else where' in the hope that being a big enough importer that jumps ship, would be enough to damage the tariffed countries economy. It's essentially an economic attack from the bottom up.

An additional reason for US tariffs being bad is that it will weaken their relationship with China, and will turn China even more towards Russia, which is presumably the whole purpose given Trumps ties to Putin. All in all the next 4 years are going to be difficult for everybody.