In the US we saw a lot of beautiful, ornamented historical buildings razed in the 50s and 60s as part of urban renewal projects, highway construction, etc.
I can think of two prominent examples off the top of my head - the original Pennsylvania Station in New York City:
source: wikipedia
and I think this NY Times quote nicely summarizes what they did:
"Until the first blow fell, no one was convinced that Penn Station really would be demolished, or that New York would permit this monumental act of vandalism against one of the largest and finest landmarks of its age of Roman elegance."
and the Metropolitan Building in Minneapolis (the entire historic Gateway district was razed actually):
source: wikipedia
Quite from Wikipedia:
"The Metropolitan Building, originally known as the Northwestern Guaranty Loan Building, is considered to be one of the most architecturally significant structures in the history of Minneapolis, Minnesota. It stood from 1890 until it was torn down starting in 1961 as part of major urban renewal efforts in the city that saw about 40% of the downtown district razed and replaced with new structures. At the time, the pending destruction of the Richardsonian Romanesque building provided a catalyst for historic preservation movements in the city and across the state. The building is considered by some to be the city's first skyscraper, with 12 stories and standing 218 ft (66m) tall."
Really breaks your heart when you see photos of these buildings
and learn how historically and architecturally significant they were. What were they thinking?? Anyways... Please, give us more examples, post pictures of the original buildings and tell us what's there now.
I'm sure there are a lot more examples from around the world especially in countries that have seen a lot of radical government changes. Russia and the former USSR come to mind and while I'm not very familiar with particular pieces of architecture that were razed by the communist government there, I do know of one - the Konigsberg castle which was razed to the ground when the area was acquired by the USSR and an ugly brutalist government building was erected there. That's right they razed this 13th century landmark castle:
source: wikipedia
and built this brutalist communist government structure in its place... arrrghh
source: wikipedia