I'm talking about cases where tariffs don't even come into play (predatory trade practices among corporate giants in U.S. history, for example).
Suppose you and I are both in the market of selling a Product. The "market price" for this Product is $5, but I decide to drive you out of business by charging $4 apiece and making up the difference by tapping into an inheritance or getting a rich uncle to subsidize my operation. My goal is to run you out of business, secure a monopoly position in our industry, and make up for all my losses by charging $6 per unit.
The problem here is that this strategy will never work, even if I am successful in driving you out of business. By underselling the "market price" by $1, I have gotten all my customers (and all of your former customers) used to the idea of buying this Product for $4. As soon as I raise the price to $6 (or even back to $5!), a substantial number of these customers are going to stop buying this Product, or at least buy as few units as possible.
An interesting point to note here is that even a company that functions as a monopoly must still engage in competitive business practices, because they have to act as if a potential competitor will open its doors tomorrow.
In the case of the industry I used in this example, it is true that you might be out of business. But in the long run you will be better off than me, because after I run you out of business I will never recover my losses. And I may even have to bankrupt myself and my stupid uncle just to stay in business through perpetual "subsidies," too.
I'm all for enforcement of existing anti-collusion and anti-monopoly laws.