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.