Is it permissible to swear to enemies, knowing that lying to them is permissible?

Lying to enemies is not permissible in an absolute sense. It is only permissible in limited circumstances, such as in times of war. Swearing a false oath constitutes another sin, which is al-yamin al-ghamus (the diving oath), and it is considered one of the major sins. In cases where lying is permissible, swearing an oath is only allowed when absolutely necessary, if the need cannot be met without it.