What does the oath "I swear by Allah Almighty that I will protect you" entail, and does looking at another woman violate it, and what is its expiation?

If you swore not to betray your fiancée, you break your oath by any action considered a betrayal, whether it is adultery or a lustful glance at another woman. Consequently, an expiation for breaking an oath becomes obligatory. Ignorance of the necessity of expiation does not waive it.