香港合約法：如中國與其他國家為敵，企業與敵國簽署的合約還有效嗎?

Unable to answer, as there are simply too many variable.

A mere declaration as an enemy is not sufficient unless there is an existing law allowing a trade ban against enemy. Then the declaration may be sufficient to excuse the obligation. If there is no existing law, an emergency law can also serve the same purpose.

For example, the U.S. has an act known as Trading with Enemy Act, banning trade to those who has the enemy designation.

Currently, Hong Kong is still under the 1 country 2 systems. As an independent WTO member, Hong Kong is not subject to Chinese trading law in term of international trading. However, if China determines such ban is warranted and impose the law in Hong Kong, it can be excused as well.

Also - the choice of law, the place of execution, etc. matters.

In short - unless the party can demonstrate an excuse to perform, the contract is not considered void, i.e. the burden of proof falls to the party who wants to cancel the contract.

大康港是一國兩制，民企不涉軍政只受本地法律法規