It appears you may be missing a word or two.
To understand the meaning, consider the following possibilities:
* "confidence of French in this undertaking seems well grounded..." In this case, "undertaking" could refer to a specific project, plan, or effort, perhaps a French effort to exploit the disunity of the British colonies.
* "...confidence of French in this, seems well grounded on present disunited state..." Here, "this" could be a more general idea or situation, perhaps a belief that the British colonies would ultimately fail because of their disunity.
Without the full context, it's difficult to give a definitive answer. Please provide the complete sentence or phrase, and I can offer a more specific explanation.