So now only two men are on trial in Germany, far from where their accusers say the crimes took place, while many more senior perpetrators, including Mr. Assad, remain in power.

Further complicating Mr. Raslan’s status is his defection to the opposition in 2012, less than two years into the war. In 2014, he even joined the opposition delegation to peace talks sponsored by the United Nations in Geneva.

Some legal advocates fear that prosecuting someone like Mr. Raslan could dissuade other former Syrian officials from serving as inside witnesses to help build other cases because they might fear facing trial themselves.

Others worry that a small number of similar prosecutions could allow European governments to feel that they are doing enough and dissuade them from broader efforts to hold Mr. al-Assad and his subordinates accountable.

“It is a good first step, an important step, but it is not going to be sufficient to fulfill the demands for justice of the Syrian people,” said Mohammed Al Abdallah, the director of the Syrian Justice and Accountability Center, which is monitoring the trial.

For Germany, the trial has become the most important of several prosecuted since 2002 on the principle of universal jurisdiction, under which national courts can try war crimes cases from elsewhere in the world.