Nash's biggest contribution was proving that in any game there's at least one point where no player can benefit by changing their strategy alone
Because the other player must react to each changing strategy and possibly change his own strategy also.
Think about the final of FIFA world cup where there will be only one country that will lift the golden cup so therefore if any team changes their tactics or strategies it affects the other team which also leads them to change their own strategy and it continues like that no team wins just by changing strategy because the other team will reciprocate
the famous Nash equilibrium thinks about traffic where all drive on the right side in America not because it's inherently better but because everyone else does
Now changing your strategy alone would be a very disastrous decision that can cause accidents and loss of life.
Now in the world economy When one country imposes import tariffs on another country, the gain is (increased revenue) for the imposing country are matched by the losses (reduced exports) for the affected country.
Will there always be a winner in a trade war ? Yes
But in a competitive world imposing trade in a country leads to a counter tariff only few countries will let you go freely most countries will reply and other factors may decide the winner like who produces more, who imports more, and who doesn't.
But the reality is there's always a winner but will that winner win by changing strategy alone? Not in all cases because humans are retaliating in nature