Can someone enlighten me what would happen if that will be US not Polish Soldiers involved in this kind of accident?
The US soldiers would be tried under the Uniformed Code of Military Justice and punished accordingly if found guilty. These soldiers in Poland are not being tried by the UN proper, they will be tried by there own people using the charges violated within the Geneva Convention.
There are circumstances where the Haig will set up a trial for war crimes, but most of the time, the group that handles war crimes will monitor the trials and not get involved unless there is a blatant attempt to not follow the punishment rules.
And what the story about Geneva convention not apply US soldiers?
This is absolute crap. We are trained over and over about the Geneva Convention and the Law of Armed Conflict and must follow those laws of face punishment. Who ever gave you this information is just bashing ignore them.
As with all things it is also dependent on the group running the Geneva Convention (for the lack of a better term) as who they will look at to be punished.