What people usually mean by sanctions is this: “If you behave contrary to my expectations, I will not trade with you”. A normal, expected answer to this is “Ok, suit yourselves, but good luck trying to heat your homes without gas, and trying to sell all those luxury goods in your oversaturated markets.”

What America means by sanctions is something else: “If you behave contrary to my expectations, I will not only not trade with you, I will also forbid you to use the agreed upon world reserve currency in trade with other countries, I will forbid international banks to do business with you under threat of blocking them out of the world reserve currency and payment systems, I will order financial institutions to confiscate your money, and forbid everyone to trade with you if they don’t want to share your fate.”

Basically, America is pressuring other countries to enforce their arbitrary sanctions, or else. This means that the sanctioned country can’t just exchange unpleasantries with America and proceed to do its thing with third parties; the third parties are threatened with exclusion from international financial system and trade, which is a dangerous thing if you have debt, and today everybody does; America controls the agencies that assess one’s creditworthiness, and if someone’s credit rating suddenly becomes “risky due to sanctions”, that would be too bad, wouldn’t it?

In a situation where the sanctioned country is militarily much weaker than America, it can’t really do much other than starve and resort to covert means of conducting trade. But in a situation where America tries to sanction a military peer (Russia or China), such sanctions are de facto a declaration of war, because it is much more effective to destroy America with military means, than to have your trade restricted, risking economic suffocation.