infinitive complements

[lexabear.livejournal.com]

If you have a present verb + infinitive complement, would you want the infinitive to be perfective, imperfective, or is it a matter of emphasis/nuance/meaning?

For instance, in a friend's journal, she asked for a translation of the sentence "I am here to defend my beloved" (she actually asked for it into Latin, but my brain automatically started providing the Russian). The Yandex gives защищать/защитить for "defend"; which one would be used in this case?