In Libertate Sociorum Defendenda

[Edmundus]

What does this mean exactly? I'm having trouble understand its grammar.

 

[Pacifica]

The best terms to use could depend on context but it means something like "in defending the freedom (alt. liberty) of the allies (alt. friends, companions, associates)".

What exactly is giving you trouble here? My guess would be that it's defendenda. That's a gerundive. Have you learned about gerundives before?

 

[Edmundus]

yes.

 

[Pacifica]

Then if you're still confused maybe read up on it again, especially about those cases where a gerundive in agreement with a noun is used instead of a gerund with a direct object. You can read the parts on the gerund and gerundive here, for example.