There were several reasons why World War I began. I will explain some of these so you can decide if the arms race was the most important factor.
Militarism, the building up of the military, was definitely a cause of World War I. Countries began to build up their army, navy, and supply of weapons because they sensed they might be involved in a future conflict. Germany wanted colonies. Since Germany recently became a united country in 1871, they didn’t have colonies. Since most lands were already colonized, Germany believed they would have to fight to get these colonies. Thus, they built up their military. France and Great Britain watched this occur, and they also built up their military to protect the colonies they already had. Militarism was a factor in the outbreak of World War I.
Another factor was entangling alliances. There were two, competing alliances prior to World War I. The Triple Alliance included Italy, Germany, and Austria-Hungary. The Triple Entente included the Soviet Union (Russia), Great Britain, and France. When Austria-Hungary declared war on Serbia, it set off a response similar to falling dominoes. The Soviet Union was allied with Serbia, so they declared war on Austria-Hungary. Now one member of each alliance was fighting each other. This caused other nations to join the war to honor and protect their alliance members.
A third cause of World War I was a growing sense of nationalism. Countries believed their way of life was superior and should be spread. These countries also believed they were better than any other country. Therefore, nobody could tell them what to do, and if a country tried to do this, they would be defeated in war. Thus, countries felt uninhibited about the actions they took.
Based on these causes, you can now make an informed decision about what you feel was the main cause of World War I.