Assuming a complete net split? Then you have chain B in China and B' outside it, each a valid ledger of a different set of transactions. When they're reunited

Assuming a complete net split? Then you have chain B in China and B' outside it, each a valid ledger of a different set of transactions. When they're reunited all the transactions only on B' get briefly invalidated as B' nodes reorganize to support the valid chain. (B is more valid than B' because it is presumably longer.). This will likely expose the network to double-spend attacks, where e.g. you take an output spent 2 weeks ago on B' and get a transaction to a B node which moves it internally in your wallet (or to a confederate). This will cause Bitcoin to forget the B' transaction and every subsequent transaction tainted by it. Your goal as the attacker is to have 1+ of these transactions give you economic value prior to your counterparty recognizing the reversal. Good news, though: a net split forking China off Bitcoin would probably result in Bitcoin's central authorities strongly recommending everyone stop processing transactions until they came back.