Why does US spend so much on their military budget?

Many of the top ranking military countries contribute or set aside a lot of money for their budget for the military. During a time when terrorism is active, North Korea has many nuclear weapons and there is much fighting in Syria, now is the time that the US is depending on their military due to so many conflicts with other countries.

Right now, the country has been fighting for years on the war on terrorism. Therefore, the US had to establish a strong budget on the military in order for them to fight the ISIS, ISIL and Taliban groups among others in order to stop them from doing senseless attacks on innocent people without any prediction that this was going to take place.