This is influenced by my experience in the North American mining industry with regards to Chinese state investment. An extreme scenario for sure, but theoretically possible?
Imagine Country A has a strong, booming economy, an internationalist culture and a totally free market. Country B has a planned economy and a clandestine desire for empire-building. State-owned businesses and agencies from Country B start to buy out companies, and buy shares in companies, in Country A.
Eventually these state agencies from B are in control of A’s largest companies, and have considerable influence over the workings of the economy in A. B’s dealings in A have always had a nationalistic undertone, and B now works to lower productivity and gradually shut down A’s economy to reduce A’s economic competition with B, so that B can reign supreme on the world stage. A’s economy is manipulated in such a way so that upstart competition is made extremely difficult, so that B’s control is secured. B’s ultimate aim might be to eventually destroy A.
Is there any defence against this from a totally free-market perspective? Or does it even really matter?
Your analogy forgets that “ownership” is at the mercy of Country A’s government. It’s just a piece of paper in some government office somewhere in County A. Anytime Country A felt like it (viz. there was a military threat) then Country A could just write another name in on those ownership documents. (This is the standard response to, “what if the Chinese bought everything”). It’s also unlikely, for instance, that every one of those companies (which presumably exhibit considerable control over Country A) are staffed significantly with citizens from Country B–obviously it will be mostly domestic workers.
Now, on a moral level, presumably Country B bought those companies from somebody and those people were compensated for it. It would be wrong for all of those sellers to sell, get their money, and then want them back.
However, if it can be objectively proven that B wanted to destroy or otherwise initiate force against A, to the point where the countries were effectively at war, then silly constructs like “ownership” don’t matter.
Another point to make here is that the above scenario would only have a chance of taking place in an extremely simple economy, such as a very small country that only had one real industry. History has in fact shown us some examples, to be sure, of countries having major problems when they heavily rely on a single industry, particularly a single commodity like oil. This problem, however, is just a tendency and is by no means deterministic.