In Hanoi, they call it the 'China factor'.
Unique among its South East Asian peers, in Vietnam no major decision is made without considering the impact on its complex relationship with its giant neighbour, according to officials and scholars.
And that even extends to the leadership of its Communist Party and government - decisions the party will finalise over the next week at its first congress in five years.
While outwardly pro-China candidates are simply not an option, amid security tensions that have seen Hanoi court its cold war enemy, the United States, neither are leaders likely to be overtly hostile to Beijing.
'The lessons of our history have created some hard realities for us,' said one Vietnamese official recently. 'We need leaders who can stand up to China and defend our sovereignty. But we also need to have a full and deep relationship with Beijing ... when you look at our borders, you will see there is no escape. We have to think constantly about balance.'
That history has left contradictions that bedevil the relationship today. Centuries of Chinese domination and Vietnamese rebellion have created cultural and political similarities as well as deep mutual suspicions - tensions that exploded into a short war as late as 1979.