You use whereupon to say that one thing happens immediately after another thing, and usually as a result of it. Mr Muite refused to talk to them except in the company of his legal colleagues, whereupon the police officers departed. used when something happens immediately after something else, or as a result of something happening
closely following and in consequence of which; "the Japanese refused to import U
closely following and in consequence of which; "the Japanese refused to import U S made cellular phones whereupon the U S imposed strict tariffs on Japanese exports"