Well the Romans had an unsung reason as well which goes for many but not all invasions - pissing their neighbors off and then getting too weak. I recall one do

Well the Romans had an unsung reason as well which goes for many but not all invasions - pissing their neighbors off and then getting too weak. I recall one documentary about Minioan archaeology - they had a practice of demanding/taking sacrifices from tributaries - the sacrificed adolescents weren't related to the other remains found. Apparently they suffered many deaths from an eruption and then when they were weakened their feared reputation backfired as their tributaries invaded and decided now was the time for vengence and to put an end to them for good. Corruption as a downfall seems to be a case of short term vested interests resulting in massive stupidity which also comes back to bite them. China weakened their navy massively over internal political disputes and even after they were forced to pay massive ransom and pardon pirates who decimated them. They even ignored the opportunity/hint as the pardoned pirates offered to sign up to be the new navy. Imperialism by western naval powers followed.