What are the reasons that Darnay's frequent trips to France result in a charge of treason in Dickens' A Tale of Two Cities?
In an ironic twist of Fate, whereas Doctor Manette is first able to get his son-in-law released from his charge of treason against the state, Manette unwittingly figures into Darnay's recapture and imprisonment. For, at first Dr. Manette is heralded as a heroic survival of political oppression in the Bastille, but upon his second efforts for Darnay, Manette is recognized as the physician who suffered by the hands of the Evremonde twins, one of whom is Darnay's father.
In Chapter 4 of Book the Third, Manette appears before the court and Darnay seems at the point of being released, when
the tide in his favour met with some unexplained check, which led to a few words of secret conference
and Darnay, ne [born] Charles d'Evremonde, is returned to prison. The secret conference that has been held involves the connection of Dr. Manette with the brother of Madame DeFarge, the young man mortally wounded after defending the honor of his sister, Therese DeFarge. Unbeknownst to Manette and Charles d'Evremonde (Darnay), Madame Defarge has knit his name into her deadly cloth and has recognized Charles after his having come to speak on behalf of the tax collector for the Marquis d'Evremonde, Monsieur Gabelle, who wrote him a desperate letter. She is in possession of a letter that Dr. Manette has written while he was in prison ( 1757-1775) that describes the atrocities committed by the Evremonds against her family. And, as the son of one of the perpetrators, Charles d'Evremonde will be held accountable.
In addition to the vengence which Madame Defarge seeks against this aristocrat named Charles d'Evremonde, the bloodlust of the revolutionaries has also risen and the "new era has begun." The Reign of Terror has begun:
The deluge rising from below, not falling from above, and with the windows of Heaven shut, not opened!
Darnay did not want to reveal his family connections to anyone in England. He had renounced his family because he disagreed with their methods, but that did not mean he would not still be considered guilty of being a member of the family, whether or not he personally wronged anyone. Since there was obvious unrest in France, both countries were suspicious of anyone going back and forth between the countries without good reason. Darnay, being a private person and trying to hide his ties to the St. Evremonde family, refused to give an explanation for these trips. Since he refused to tell, it was assumed that he was up to no good.
There were many spies going back and forth from England to France. The trip was an especially dangerous one if you were French and living as an expatriate in England. Darnay also realized that revealing his family connections would only further condemn him.