According to the Quran they have the right to fight those who are fighting them. This does not include fighting the civilians of course.
In this regard we have to remember that war in the real Islamic jurisprudence is the last option, it is an exception while the peaceful struggle is the general rules. You fight if you have to, if there is no peaceful way else to solve this problem.
Peace is the best Islamic solution in dealing among individuals and in dealing between countries. Peaceful efforts include forgiveness and reconciliation. God says in the Quran:” And the servants of the Almighty Lord are those who are walking in the earth modestly, and if the ignorant insult them they say: “peace”. [25: 63]. Also He says “Then forgive them and say: “peace”. They will come to know. [43:89].
Peace is the best solution because by peace you are addressing the noble values inside your enemy, giving him the chance to correct his bad deeds. But by the war you are invoking his bad side. After thousands of centuries and hundreds of wars mankind realizes lately this Quranic value of peace while the fanatic Muslims are denying it.
In November 1987, I was in the notorious prison of Torah near Cairo because I advocated the real Islamic values of peace, tolerance, justice, democracy and human rights proving that the so –called Sunna has the culture of bigotry and fanaticism. In prison, the intifada or the rising of the Palestinians had begun. The prison was full of Muslim fanatics who were eager to see the Intifada changed from stoning the Israeli soldiers to fighting them by machine guns. I was so scared to speak up, but in my dreams in that time, I hoped that the Palestinians should demonstrate peacefully and silently with white flags saying “We want freedom. We want justice. We want peace.” By this way they would get their state and save all these lives from the two sides.
By the Salafi Wahaby culture, Muslims forget the high Islamic value; Peace. In the same time there are great leaders who were aware of the value of peace as the perfect way in resistance, and by peaceful means they got freedom for their people. Mahatma Ghandi of India addressed the British consciousness directly by his peaceful humble struggle and got the independence for India, but the fanatic Indian Muslims have divided India and created Pakistan as one of the biggest mistakes in the last century. Marten Luther King, by his peaceful struggle got the civil rights for the African Americans refusing to use guns because the other side has more guns. Nilsson Mandela in his prison for 30 years defeated the whit regime in his country because he is a man of peace. Those are great men and great Muslims also in the realm of peace.
In the other hand, the first struggle did by the P.L.O was destroying a school bus full of children in Israel, then the other kind of struggle was hijacking the civilian aero planes. The current struggle is by the suicide bombers which is advocating by the Salafi fanatic culture and its leaders and their verdicts on line.