It was vulnerable because most of the Rome lands were captured and so when a threat came the conquered place joins the threat.
There were mountains, barriers, and lots of water around to prevent invasion and disease
This is what i would put East Asian policies toward the United States will be driven strongly by the uncertain regional security environment, the nascent revival of regional economies after the Asian economic crisis, and trends in international politics and norms that affect East Asian authoritarian and democratic governments differently but underline strong regional nationalistic pride and assertiveness
