What might make government officials decide to spend additional money on social security instead of the military? what would make government officials decide that the marginal benefit of social security spending is greater than the marginal cost of military spending? ]

Additional money on Social Security can actually lead to an increased security in the country, something the military also wants to achieve.

Specifically, it will decrease a number of people in desperate positions who are more likely to commit crimes.

So for example, decreasing poverty can be more effective as a anti-terrorism measure because it decreases people's desperation, which often leads them to join gangs and terrorist groups.