If a Muslim state attacks another Muslim state then defending the country obviously is Jihād for the Muslims of invaded country. In such a situation, what should be the stance of the Muslim soldiers of the invading army? What are they supposed to do if they find before them their Muslim brethren?
Today, the only basis of Jihād endorsed by Islam is one that is against oppression and persecution. This is irrespective of the fact whether this oppression is being perpetrated by Muslims or non-Muslims.
As regards the situation you have referred to, it bears many possibilities. A Muslim is supposed to support the truth and support any move against oppression. It is obvious that a Muslim country is not always on the right. When the invaded Muslim country is not on the right and the soldiers of the invading army know that the country in question has a wrong standing, then their fight should be considered Jihād. If, however, they know that the other country is fighting for a just cause and their leadership is forcing them into committing oppression, they can leave their service in favor of the right cause. They must be ready in this case to face the consequences as well.