From 1917: War, Peace, and Revolution, by David Stevenson (OUP Oxford, 2017), Kindle pp. 297-298:
By the end of 1917 most of the world’s population had entered a state of belligerency. Even during the Napoleonic Wars this situation had no precedent. Two impetuses to the process came from the unrestricted submarine campaign and from American entry. The first threatened death and destruction to almost every country; the second made neutrality less attractive and joining the Allies more so. Yet the new belligerents made their own decisions, which were frequently contested. In China intervention led to civil war, and in Greece to something close to it; in Brazil it prompted civil disorder and repression of the German-Brazilians. In China, the issue became embroiled with the contests between Duan and Li and between the northern Chinese warlords and the Guomindang. Intervention became a gambit in a domestic struggle, with Duan holding the advantage. Brazilian public opinion was always pro-Allied in tendency, but it took the submarine sinkings to create a Congress majority for belligerency. Finally, in Siam the government had no legislature to contend with, and once the king insisted on intervention his foreign minister assented.
None of the four countries envisaged an all-out struggle, which makes their interventions easier to comprehend. So does US entry, which made the Allies more likely to win. Indeed, America also initially envisaged a limited commitment, but unlike the other new arrivals it subsequently expanded it. China, Brazil, and Siam were remote from the Central Powers and therefore ran little risk. Greece ran a bigger one, as a fighting front ran through its northern territory, and of the four it made the biggest military contribution. But the costs and risks should be set against the prospective gains. For Brazil these were primarily economic. For Siam and China the additional incentive was gaining traction against the unequal treaties, the Chinese being particularly focused on the Shandong lease. In Greece Venizelos wanted Bulgarian and Turkish territories that might support a glittering future in the Eastern Mediterranean and Aegean. The prize all sought was a voice in the peace settlement.
These objectives would be satisfied unequally and tardily; and in Greece’s case scarcely at all. But the widening of the war through new interventions weakened European pre-eminence. Siam and China challenged the unequal treaties in a manner impossible before 1914; Chinese nationalism strengthened and became more anti-Western; Brazil and other South American countries turned away from Europe. China’s intervention was determined more by Japan than by the European Allies or the United States. Moreover, the war’s prolongation undermined not only informal European dominance in East Asia but also formal control elsewhere. This was most evident in the August 1917 Montagu Declaration, promising ‘responsible government’ in India, the grandest empire’s biggest possession. But if European control was under challenge in Asia, it was still expanding in the Middle East, and 1917 was the decisive year for establishing British authority over Palestine and Iraq. These developments too would figure among the lasting consequences of these crowded months.