These nations are the core of the UN Security Council:
* United States is a federal presidential constitutional republic.
* United Kingdom is a unitary parliamentary constitutional monarchy.
* France is a unitary semi-presidential constitutional republic.
* Russia is a federal semi-presidential constitutional republic.
* China is a nominally Marxist-Leninist single-party system nation.
Excluding China, how are these nations liberal democracies?
I often heard people say that if the Roman Empire hadn't fallen to the barbarians, that our world would be a different place today. This I think is a fallacy. First, the Roman Empire in the east became the Byzantium Empire, and they certainly didn't advance the progress of humanity's technology. The people of Islam advanced humanity's technology, and it was their embrace of modern technology that led them to a successful win in the Battle of Constantinople in 1453. (During the same time, the people of Asia were making advances in technology (warfare, printing, etc) which aren't as well reported because of a bias in Western nations' history courses.) Secondly, the culture of Rome depended on a large illiterate population governed by a small literate aristocracy. If the technology benefited the aristocracy's interests, it was supported by an investment of money and was later spread throughout the empire. If it didn't, the technology was introduced and then promptly abandoned.
I don't think we will ever know how much the Greeks and the Romans knew about science or mathematics, nor what was actually attempted because of a dearth of literary sources.
I think it is wrong to place the majority of the lost of knowledge on the barbarian invasions. Christians share a responsibility in the loss of knowledge. They burned books deemed heretical to the faith, and, in the Byzantium Empire, they showed a preference for Greek writings, thus a great many Latin works that had survived were subsequently lost. And, finally, the major reason for the loss of knowledge must fall on the aristocrats who didn't educate the populace, and, when the aristocrats were killed, there was no one left to educate the populace in making concrete. This is to me is the biggest difference between the Roman Empire and the empires of today - everyone can be educated in a public school.