Empires with their immense military strength remained fragile to civil wars, economic decline and a changing political environment internationally. In 220 AD Han China collapsed into warring states while the European Roman Empire began to suffer from turmoil in the Third Century Crisis. In Persia regime change took place from Parthian Empire to the more centralized Sassanian Empire. The land based Silk Road continued to deliver profits in trade but came under continual assault by nomads all on the northern frontiers of Eurasian nations. Safer sea routes began to gain preference in the early centuries AD