Trade wars do hurt. But what else can (should?) we do about things like China's currency manipulation?
I'm not sure it is (any more).
The yuan is not so free-floating as the dollar maybe, but certainly its value is dependent on the market. China dearly wants to be one of the big boys, including on the currency market. To be big in the market, the market must trust its currency, and the more it manipulates its currency the more it forfeits that trust. Currency manipulators usually work by running a surplus, and using that money to buy foreign currencies. Buying foreign currencies (increasing demand) drives up the value of those currencies, and so keeps the home currency weak. But China doesn't do that. Thus I think the designation by Trump of it as a currency manipulator was just political theatre, whipping up some good old xenophobic sentiment and showing off how tough he was.
Broadly, I have no objections to the USA engaging China in a trade war. It has reasonable grounds to want to rethink things: China does play the system to give itself an "unfair" advantage. However, the presentation of the trade renegotiation by Trump to the USA is a just a load of complete guff. The tariffs are pretty much useless, they're just a bogus attempt to look tough to his domestic audience, too. What the USA really wants is to stop China extracting IP from US companies that do business there, and to reduce China's protectionism, so increasing non-Chinese companies' ability to do business and invest in China. This isn't about benefitting US workers anything like as much as it is benefitting Wall Street. After all, they own the companies hoping to open Chinese operations (in China, thus with Chinese workers, obviously) and they'll be doing the investing.
The irony of course is that Trump wants to hammer China for not playing fair, but Trump's own vision for the USA was that it shouldn't play fair either. This is the nature of the international order: everyone agrees to do things a certain way. As Trump was busy attacking China for not playing by the rules, he was also busy attacking the rules (WTO, UN, etc.) so he didn't have to play by them either. There's a hypocrisy in there, to complain that others are doing what you want to do.
The Trumpian image was the US as a great big unilateral power that would leverage its economic might to secure advantages for itself over weaker trade partners. All very pre-WW2 thinking. What this is mostly likely to end up with is heavy protectionism and trade barriers, slower growth, across the whole world. The concept of the post-WW2 world was that free trade under established rules would forment both peace and faster development, precisely to avoid the sort of problems that occurred after the Great Crash of 1929, where all the countries pulled the shutters down to protect themselves, and mostly just magnified the depression for everyone instead.