According to international relations scholars, alliances make other countries afraid and can (under certain conditions) make war more likely. You could argue that this happened, for example, in WWI.
Countries tend to enter into alliances to keep themselves safe. However, when they do this, they make other countries afraid. For example, imagine if Country A and B are low-level enemies. They aren't about to go to war, but they are suspicious of one another. So now A enters an alliance with Country C. What will country B think? They will worry that A and C will team up and attack them. They may want to attack first to try to start and win a war before A and C are ready.
In WWI, alliances between France and Russia (among others) made Germany worried. This is one reason that they acted so aggressively at the start of this war.