Complex sentence: Chattopadhyaya's efforts--along with a letter from the Kaiser--convinced Pratap to lend his support to the Indian nationalist cause, on the condition that the arrangements were made with the Kaiser himself.
Split into simpler sentences: Chattopadhyaya's efforts, along with a letter from the Kaiser, were able to convince Pratap to lend his support to the Indian nationalist cause. Pratap agreed to lend his weight on the condition that the arrangements were made with the Kaiser himself.