～上は — Now That the Die Is Cast

引き受けた上は、最後までやり遂げます。

Hikiuketa ue wa, saigo made yaritogemasu.

Now that I've taken this on, I'll see it through to the end.

引き受けた (accepted) + 上は; because the task was accepted, firm resolve follows.

留学すると決めた上は、しっかり準備しよう。

Ryuugaku suru to kimeta ue wa, shikkari junbi shiyou.

Now that I've decided to study abroad, let me prepare thoroughly.

決めた (decided) + 上は leads to a determined course of action.

約束した上は、必ず守るべきだ。

Yakusoku shita ue wa, kanarazu mamoru beki da.

Since you've made a promise, you should absolutely keep it.

約束した (promised) + 上は produces an obligation expressed with べき.