Is balance of power a cause of war, condition of peace, or both?

I would say that balance of power is both a cause of war and a condition of peace.  A balance of power can bring about peace, but it can also cause war when a country or countries tries to break up the balance that currently exists.

Balance of power can certainly bring about peace.  There was balance of power in the “long 19th century” and peace in Europe because of it.  We can say that there was balance of power between the US and the USSR during the Cold War.  Between those two countries, at least, there was peace.  When there is a balance of power, countries know that they cannot gain by going to war.  They will not go to war because they are likely to lose without gaining anything.  In this way, balance of power can bring about peace.

However, balance of power can also cause war.  When there is a balance of power, there is always the possibility that some country will try to get stronger so as to break the balance.  When this happens, the other countries will typically go to war to try to prevent the first country from breaking the balance.  This, in a sense, is what happened in WWI.  There was a balance of power between European countries, Germany tried to increase its power to break that balance, and the other European countries fought to prevent this.

In these ways, balance of power can act both as a force for peace and a condition that can bring about war.