If free trade is mutually beneficial, what is the logic behind conquering countries for their resources? Japan in China in the 30s for example, when the Japanese invaded Manchuria to get the mineral resources, why not just buy them? Is conquest cheaper? Did it make the average Japanese citizen better off? And any other examples/countries would be fine to prove the point, I'm not only curious about Japan specifically
I don't want to be picky - I understand the distinction you're getting at - but I want to point out that imperialism and free trade are not incompatible. In fact, the imposition of free trade can be an imperialist objective. Look at the Opium Wars, for example, which were very much about access to Chinese markets.
Look at the British Empire. From the 1840s onwards Britain became a strong advocate of free trade. This was also when the British Empire was growing to its zenith. However free trade was advantageous to Britain, because it was dominant in industry, commerce and shipping, and the terms of trade were good. In other words, the capacity of Indian merchants and industrialists to exploit the British market was much less than the British exploiting Indian markets. (This is not to say that the way trade was structured was in any way fair.)
The British Empire was essentially turned into a giant low-tariff zone that was only properly dismantled after the UK joined the EEC. Even today there are some people who say we should leave the EU and return to trading with the empire.