What did Siam give up to stay independent?

Its edges. Between 1867 and 1909 Siam ceded territory that is now much of Laos and western Cambodia to France, and four Malay states to Britain - perhaps a third of what it had claimed. The strategy was explicit: give up the periphery, keep the core, and stay the buffer both powers preferred to a shared border.

It also gave up a good deal of sovereignty without giving up the name. The unequal treaties beginning with Bowring in 1855 imposed extraterritoriality - Europeans in Siam were tried under their own consular courts, not Siamese law - and capped import duties at three per cent, so the country could not set its own tariffs. Those were not fully undone until the 1930s.

Kings Mongkut and Chulalongkorn spent the same decades modernising deliberately and fast: abolishing slavery over a phased twenty-year process, reorganising the bureaucracy on European lines, building railways and telegraphs, and sending princes to be educated abroad - on the reasoning that a state which looked civilised by European standards was harder to justify colonising. Never colonised is true. Never coerced is not, and the two get run together too easily.