His name had been upon the list of the proscribed for some time; but the end was precipitated by an act of his young son, Louis Philippe, then Duke de Chartres, and aide-de-camp to Dumouriez, who was defending the frontier from an invasion of Austrian troops.

After the execution of the queen, Dumouriez refused longer to defend France from an invasion the purpose of which was to make such horrors impossible.

He laid down his command, and, with his aide, Louis Philippe, joined the colony of exiles in Belgium, while the Austrian troops were in full march upon Paris from Verdun.
This was treason--whether justifiable or not this is not the place to discuss.
Philip Egalite knew that he no longer had the confidence of the leaders, and that they also knew that he was an aristocrat in disguise.
So when this defection of Dumouriez came, and was shared by his own son, he tried to get out of the country.

He was arrested at Marseilles, brought to the Conciergerie, that half-way house to the scaffold, and was soon following in the footsteps of his king and queen, through the Rue St.Honore, passing his own Palais Royal on his way to the Place de la Revolution.
The Revolution, beginning with a patriotic assembly, in a measure sane, had made a rapid descent, first falling apart into Girondist and Jacobin, moderate and extremist, the Girondist with a shudder consenting to the execution of the king.

Then, the power passing to a so-called "Committee of Public Safety" and a Triumvirate, in order to sweep away the obstructive Girondist; and then an untrammelled Terror, in the hands of three, and, finally, one.