Fighting on foreign land

Do we have the right to fight on foreign land even if it was for good cause well the answer is yes and no because if you remember when Brits fled the country and joined Isis to fight for them they were wanted back in Britain and had arrest warrant ready for them. It was against the British interests to join the Isis and fight for them. However it's a right for the British to fight in Syria with against the Syrian Rebels where it all matters for the interest of the British people. So fighting on foreign land is a risk and reward.