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Trump had taken a position early on that he did not want the US to be the world's policeman (this must have really ticked off John Bolton) or the world's unconditional candyman when it came to foreign aid. Using Quid Pro Quo for foreign aid is not a new idea.
It's not, but the examples we're aware of have been done openly, and they've involved matters such as the quality of governance and human rights in the countries involved. In this case the quid pro quo was pursued covertly in pursuit of domestic political advantages for the US President personally.
Several current Democratic candidates have suggested using a Quid Pro Quo: military aid for stopping settlements in the West Bank.
This concerns a matter of international law : the settlements are on expropriated land in militarily occupied territory, and such expropriation is illegal under the Geneva Conventions. Withholding military aid from a country until it stops breaking the Geneva Conventions is surely legitimate, and of course would occur in the light of day.
Not withholding military aid in such circumstances is much more questionable. Any actual examples you have in mind can probably be as easily justified.
When it comes to governance in the Ukraine (about which there are legitimate concerns), Zelensky's government has already committed publicly to examining investigations which have been discontinued with a view to establishing
why they were discontinued. That would include the Burisma investigation (but of course
not an investigation into where George Soros has hidden the DNC server). Zelensky has done the right thing.
It seems it would have been an easier road trying to show the legitimacy of the object of the Quid Pro Quo.
That would be impossible, since the object of the quid pro quo was an equivalent to Clinton's Emails to use against Biden.
Russian/Ukrainian corruption has been a long time concern of the State department and previous administrations.
Why do you lump those together? Ukraine is not Russia and Russia is not Ukraine. (Best ignore the Crimea for simplicity's sake.)
Asking Ukraine to check out a specific situation in a known corrupt company to see if there was corruption was certainly better than asking for approval of one of his golf course resorts.
Asking Ukraine to investigate a specific case
because the investigation could be spuriously linked to Biden is not legitimate. It just isn't. Trump isn't concerned about corruption in Ukraine (nor in Russia, of course, nor, indeed, in the US).
Back to the West Bank example. What if Elizabeth Warren (who could be part Palestinian, who knows) was President and wanted to implement the military aid for stopping settlements in the West Bank Quid Pro Quo. If she had family or property interests in the West Bank would she have to stop and say: I'm sorry Palestinians, you will have to wait another 4 or 8 years for this action, it would not be proper for me to do it?
She could divest herself of her own property interests and be open about her family's interests.
We don't know what Trump's property interests are in Ukraine, and as to him ever being open about his own or his family's financial interests anywhere ever, fuggedaboudit.