Complex sentence: The British regarded Thailand as having been partly responsible for the immeasurable damage dealt upon the Allied cause and favoured treating the kingdom as a defeated enemy, but the Americans had no great sympathy for British and French colonialism and decided to support the new government.
Split into simpler sentences: The British regarded Thailand as having been partly responsible for the immeasurable damage dealt upon the Allied cause and favoured treating the kingdom as a defeated enemy. However, the Americans had no sympathy for British and French colonialism and supported the new government.