Well, we can’t reach an agreement as to not bomb each other at times… I think (not sure but could check) that at any given time in the past centuries there were at least 2 nations at war with each other. Also different political systems ranging from liberal democracies like the West to theocratic totalitarian regimes like Iran. These don’t typically have the same approach to different issues so I don’t think they could agree on a common currency and monetary policy.
Even if you hypothetically get past all that in your project of establishing a world currency…then, who is in charge? who sets interest rates? who decides if it’s favourable to devalue money? Let’s say you make a democratic world bank UN style where each country votes: in this case would a “one country one vote” system be fair? The UK has the same GDP as te entire continent of Africa…would they have an equal say?
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