Jihad literally means "struggle" or "resisting". Within an Islamic
perspective, it is "striving in the way of God (al-jihad fi sabil
Allah). [Copied from the tag title on this site]
Women are encouraged to participate in jihad (Bukhari 1861):
Narrated Aisha (mother of the faithful believers): I said, "O Allah's
Messenger (ﷺ)! Shouldn't we participate in Holy battles and Jihad
along with you?" He replied, "The best and the most superior Jihad
(for women) is Hajj which is accepted by Allah." `Aisha added: Ever
since I heard that from Allah's Messenger (ﷺ) I have determined not to
Jihad in our times does, in almost all cases, does not mean to fight with weapons against an enemy. Which of the recent or wars was jihad? Which of the ongoing wars is jihad?
Today, other efforts are needed to do the will of God, and both women and men are called to do this effort.
Women were not fighting in the war Allah's Messenger ﷺ had to fight. The passage above indicates that Muhammad ﷺ rejected the request of Aisha to participate in the battle. The fact that Khawla bint al-Azwar participated in a war under a Muslim emperor does not mean that this is covered by Islam.
A rejection of aggressive war is not anchored in Shari'ah. This would not have been possible as the law was developed under the rule of emperors who diligently (and quite successfully) waged war. I said «under» because the Shari'ah law, although it is meant to be a ruling only based on the rules of God and the example of His Messenger ﷺ, it was the law of the state and under protection of the emperor. There was no supranational instance lower than God himself that could condemn or inhibit the action of the state.
Nevertheless, waging war (aggression) is against the will of God:
O you who believe! When you journey in the way of God, investigate,
and do not say to him who offers you peace, “You are not a believer,”
aspiring for the goods of this world. With God are abundant riches.
You yourselves were like this before, and God bestowed favor on you;
so investigate. God is well aware of what you do.
And fight in the cause of God those who fight you, but do not commit aggression; God does not love the aggressors.
Defense is not against the Will of God:
Permission is given to those who are fought against, and God is Able
to give them victory. Those who were unjustly evicted from their
homes, merely for saying, “Our Lord is God.” Were it not that God
repels people by means of others: monasteries, churches, synagogues,
and mosques—where the name of God is mentioned much—would have been
demolished. God supports whoever supports Him. God is Strong and
In reality, once war is waged, the decision whether a war is aggressive or defensive is often unclear. The war Khawla bint al-Azwar faught was not the first fight between the Caliphate and the Byzantine Empire but a counter-offensive to a counter-offensive in a war that had initially been waged by the Caliphate. It was declared jihad but it wasn't.
God does not love the aggressors.
Gospel of Matthew, 5:9
Blessed are the peacemakers, for they shall be called sons of God.
Both sides should have known.