For a century and a half, HSBC Holdings steered a neutral course through the messy geopolitics of east versus west. Now a public signal of the bank’s support for China over the future of Hong Kong has set off a storm of criticism.

At a plastic table outside of a Hong Kong metro station, the bank’s Asia chief, Peter Wong, recently signed a petition backing a security law China is preparing to impose on the city. Opponents say the law would undo the territory’s autonomy, threatening its Western-style pillars of free speech and independent courts. Beijing says the law is necessary to restore order after a year of protests.