Complex sentence: At the time of the Declaration of Pillnitz, Brissot headed the Legislative Assembly: the declaration was from Austria and Prussia warning the people of France not to harm Louis XVI or they would "militarily intervene" in the politics of France.
Split into simpler sentences: At the time of the Declaration of Pillnitz, Brissot headed the Legislative Assembly. The declaration was from Austria and Prussia warning the people of France not to harm Louis XVI or they would "militarily intervene" in the politics of France.