I know this question looks low level, but it's been something that has had me stumped for a while and I haven't been able to find any answers about it(probably because it's simple and I haven't realised it). My problem isn't with grammar or vocabulary.
Mainly this problem appears in narrative of novels etc rather than in actual speech. I was doing some scanning of the usage of the modal た briefed in some papers such as http://www.cocopb.com/NichiNichi/journal_1/%E3%82%A8%E3%83%B3%E3%83%88%E3%83%AA%E3%83%BC/2010/5/5_%E6%97%A5%E6%9C%AC%E8%AA%9E%EF%BC%8F%E6%97%A5%E6%9C%AC%E8%AA%9E%E6%95%99%E8%82%B2%E7%A0%94%E7%A9%B6__1__2010_files/2010_1_sadanobu.pdf and this did clear up some things about how the た form is used, but I still don't quite understand.
Take these examples below
紛れもなく、もう限界だった。今までのダメージ蓄積もそうだが、先の一撃で左腕に力が入らない。これではまともに剣すら振るえない。構えるだけが精一杯だ。
俺と彼は、戦いを通じて確かに分かり合った。その関係は、言葉に出さなくても、間違いなく『友人』といえるものだった。
Why in these sentences would だった and not だ be used(in a narrative)?
From what I understand in the usage of だった like this in narratives is that it is similar to what we would call "past" tense in English.
Take for example the line 彼は友達だったーぃえ、友達です
From my eyes the 友達だった part refers to a point in time where you felt this, but then he corrects this by saying 友達です
to show that is still the current situation. But if we look at the above sentence where it says 間違いなく『友人』といえるものだった, why would this phrase be used in the た form if they had only very recently done what was described in the rest of the sentence and were not friends before this.