What is the impact of the formation of Germany on the countries that joined European Union?
I assume that you are asking about impacts of Germany on the European Union, not the impact of Germany on the countries of the EU through things like World War II.
If this is so, the formation of Germany has had both positive and negative impacts on the European Union. The creation of a united Germany has made it the most economically powerful country in the EU. This has been very beneficial to the EU in many ways. It has allowed the other countries of the EU to benefit because they can trade with Germany freely and can send workers there without limits.
However, the formation of Germany is also a problem for the EU countries in some ways that we can see today. Specifically, Germany is seen by many of the other countries as being too demanding in the current crises. Since Germany has been something of a model of fiscal rectitude in recent years, it expects other countries to act the same. It wants to require them to commit to serious reforms before it helps to bail them out. This may be the proper thing to do, but it is also the source of much conflict within the EU.