Peace Tpofto, I agree with you. Chauvinism may appear and actually appears in many cultures and religious circles. Racial and ethnic motivations has been a major cause of bloodshed all over the world.
Yet, I don't agree with you on Umar r.a., someone known with his sense of justice. True, Umar r.a. sent armies against Byzantium and Sasanids and to other places. So did Muhammad pbuh. Muhammad pbuh send an army to challange Damascus governor of Byzantium empire when his envoy was massacred by that governor. A short war broke in a place called Muta.
Here I will try to outline Islam's view on war, with excerpts from Qur'an.
Islam does not take a passive approach in matters of war. War is the means for keeping justice and for self defense.
[Qur'an 22:39] Permission (to fight) is given to those upon whom war is made because they are oppressed, and most surely Allah is well able to assist them;
[Qur'an 4:75] And what reason have you that you should not fight in the way of Allah and of the weak among the men and the women and the children, (of) those who say: Our Lord! cause us to go forth from this town, whose people are oppressors, and give us from Thee a guardian and give us from Thee a helper.
When a just war breaks out, you pursue it until you win or get defeated or you reach a settlement. It is important to abide by the settlement if you sign it. Otherwise it wont be a just but hostile cause.
[Qur'an 9:12] And if they break their oaths after their agreement and (openly) revile your religion, then fight the leaders of unbelief-- surely their oaths are nothing-- so that they may desist.
Islam does not take a passive approach even when social order is concerned in between believers.
[Qur'an 49:9] And if two parties of the believers quarrel, make peace between them; but if one of them acts wrongfully towards the other, fight that which acts wrongfully until it returns to Allah's command; then if it returns, make peace between them with justice and act equitably; surely Allah loves those who act equitably.
Justice is considered the foundation of any state, society, family, any existent order. It perhaps precedes Mercy in that sense. God says He can forgive everthing but not our oppressive and unjust behaviour against others. He wants us not to come to Him with the burden of rights of others on our shoulders. He says He will not forgive us on behalf of the oppressed, until the oppressed takes his rights back. His Mercy is limitless but is His justice too.
[Qur'an 10:44] Surely Allah does not do any injustice to men, but men are unjust to themselves.
One thing that can spoil the just cause of a war is the way you carry it out. Means are as important as the goal.
[Qur'an 2:190] And fight in the way of Allah with those who fight with you, and do not exceed the limits, surely Allah does not love those who exceed the limits.
Maybe the problem with Christianity lies here. Since there are not defined limits for keeping justice and carrying out state affairs, Christians pursued it with their own desires. And that resulted in atrocities. Who can say "loving" Christians did always kept with the love. Even the Church became champion of most brutal wars.