までだ / までのことだ

までだ and までのことだ present an action as all there is to it, with nothing more to discuss. The two mean the same, までのことだ with a little more weight, and the polite form is までです. What they mean depends on the verb form in front: the dictionary form gives "I'll just do X", the た-form gives "I only did X".

After the dictionary form: "I'll just…"

The verb names a fallback the speaker will calmly take if the plan fails. A condition with ば, たら or なら usually comes first, and the tone is resolve rather than disappointment.

After the た-form: "I only…"

The verb names something already done, and the pattern plays it down: I did that and nothing more. It answers a reaction, so the subject is nearly always the speaker. Against criticism it defends. In the middle of a sentence it becomes までで.

Against thanks or praise it is modest: it was nothing.

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Both forms are firm and bookish. In conversation, the fallback is きごう〜ばいい or きごう〜しかない, and the playing-down is きごう〜ただけだ, which states the fact without the defence.