The Crown Prosecution Service (CPS) has chosen to warn of the possible consequences for those who decide to go to Syria to oppose the Assad government - see The Muslim Issue and also BBC - Britons returning from Syria face arrest, says Police Chief 25th January 2014. The concern is that young people who do this may have become "radicalised" and thus become a threat within the UK.
In the UK,
terrorism is defined on the Terrorism Act 2000 section 1.
In this Act “terrorism” means the use or threat of action where -
Action involving serious violence against a person or serious damage to property are examples of matters within subsection 2. Subsection 4 extends the word "action" to actions outside the UK. Any reference to "property" includes property outside the UK. Also, the word "government" includes the government of a country other than the UK.
Thus, the reach of UK terrorism law is very wide. No matter how evil or detestable the regime being opposed, those opposing it may find that they have committed a terrorist offence.
The reason(s) for the CPS announcement are not entirely clear but it is interesting that the announcement came just before the publication of the Criminal Justice and Courts Bill which will increase the penalties for certain terrorist offences to life imprisonment. For example, the Bill contains this clause:
In section 54(6)(a) of the Terrorism Act 2000 (penalty on conviction on indictment of offence involving weapons training for terrorism), for “imprisonment for a term not exceeding ten years” substitute “imprisonment for life”.
The Terrorism Act contains provisions for extra-territorial jurisdiction
There is an Independent Reviewer of Terrorism Laws - Mr David Anderson QC
7th February - Both sides in Syrian conflict might be guilty of war crimes according to the United Nations
Last moments of the British jihadist - Daily Mail 7th February
11th February - George Monbiot - The Guardian - Orwell was hailed a hero for fighting in Spain. Today, he'd be guilty of terrorism
17th February - Independent - Growing fears over Britions radicalised in Syrian conflict