A 44-year-old man from Watford has appeared at Westminster Magistrates' Court after being charged by police under the Terrorism Act 2006.
Tahir Bhatti, of Clarke Way, has been charged with assisting a 26-year-old man from London to commit terrorist acts.
His co-accused Imran Khawaja, of Southall, London, has been charged with preparing to commit terrorist acts and receiving training in the use of firearms.
He is also charged with attending a place for terrorist training.
Bhatti will appear at the Central Criminal Court on June 27.