上は details
write + (past) + now that
kaita ueha
now that; given that — a settled fact demands a resolve, stated in formal or declamatory register
こうなった上は、最後まで戦うしかない。
kou natta ue wa, saigo made tatakau shika nai.
Now that things have come to this, there's nothing to do but fight it out to the end. neutral
一度お引き受けした上は、途中で投げ出すわけにはまいりません。
ichido o hikiuke shita ue wa, tochuu de nagedasu wake ni wa mairimasen.
Having accepted the task, I cannot possibly abandon it halfway. formal
Examples
now that; given that — a settled fact leaves no room to turn back, stated in a formal or declamatory register
こうなった上は、最後まで戦うしかない。
kou natta ue wa, saigo made tatakau shika nai.
Now that things have come to this, there's nothing to do but fight it out to the end. neutral
一度お引き受けした上は、途中で投げ出すわけにはまいりません。
ichido o hikiuke shita ue wa, tochuu de nagedasu wake ni wa mairimasen.
Having accepted the task, I cannot possibly abandon it halfway. formal
委員長をお引き受けした上は、誠心誠意、職務を全うする所存でございます。
iinchou o o hikiuke shita ue wa, seishinseii, shokumu o mattou suru shozon de gozaimasu.
Now that I have accepted the role of committee chair, it is my intention to carry out my duties with complete sincerity. formal
Formation
{V-ta}うえはattaches to た-form
Notes
announcement, a vow. Sounds staged if used in ordinary conversation.
Related patterns
Similar
以上は sits comfortably in ordinary firm speech and print alike; 上は is confined to formal writing and declaimed speech — a ruling, a formal announcement, a vow — and reading it in casual conversation sounds staged where 以上は does not.
からには is fully productive on a non-past premise — 住むからには, 引き受けるからには; 上は is essentially confined to a settled た-form fact (こうなった上は, 引き受けた上は), since its non-past reading survives only in the frozen literary かくなる上は that ordinary verbs cannot fill.
Lookalikes
The same 上 with opposite logic depending on the particle — 上は (now that / given that) draws a firm conclusion FROM an already-settled fact (こうなった上は、後には引けない); 上で (after doing / in the course of) merely sequences one deliberate step before another (相談した上で決める) with no resolve implied — reading 上は as 上で drops the obligation the sentence is making.