Summa Theologica, II II, Q. 40, A. 2, ad 3:

Ad tertium dicendum quod, sicut supra habitum est, omnis potentia vel ars vel virtus ad quam pertinet finis habet disponere de his quae sunt ad finem. Bella autem carnalia in populo fideli sunt referenda, sicut ad finem, ad bonum spirituale divinum, cui clerici deputantur. Et ideo ad clericos pertinet disponere et inducere alios ad bellandum bella iusta. Non enim interdicitur eis bellare quia peccatum sit, sed quia tale exercitium eorum personae non congruit.

Which in English is:

To the third (objection) it must be said, that, just has has been had above, every power and/or art and/or virtue to which an end pertains, has (an ability) to dispose concerning those which are toward that end.  Moreover, carnal wars among believers are to be referred to an end as to the spiritual good of the Divine, to which the cleric is deputed.  And for that reason it pertains to clerics to dispose and induce other (men) to fight just wars.  For it is not interdicted to them to fight because  it be a sin, but because such an exercise does not befit their persons.