After the Second World War, European countries realized that if they had a union of some sort, they would be less likely to go to war with each other. Therefore, in 1979, they decided that they would link their currencies together at a fixed exchange rate under the Exchange Rate Mechanism (ERM). Because Germany had the strongest, countries in the ERM system agreed to fix their currencies to the Deutschmark within 6% of the agreed rate.

https://ngcbgroup.com/blogdetails/how-george-soros-almost-broke-the-boe