An undersea cable was laid to Bombay from England in 1863, which was already linked to North America. By 1854, there were 800 miles of telegraph lines between Calcutta and Agra which was further connected to Bombay and Madras. Calcutta to Bangkok is another 1000 miles of much, much slower communication, but at least for part of the Civil war there was at least the potential to exchange messages relatively quickly.