"Hashem spoke to Moses in the plains of Moab, by the Jordan at Jericho, saying, Speak to the Children of Israel and say to them, When you cross the Jordan to the land of Cana'an you shall drive out all the inhabitants of the land before you.…. But if you do not drive out the inhabitants of the land before you, those of them that you leave shall be as pins in your eyes and as thorns in your sides and they shall harass you upon the land in which you dwell."
This important warning given over to the Jewish people at the entrance to the Land of Israel was reinforced by Joshua while the Jews were crossing the Jordan River.
The Talmud relates to us that the Jordan River miraculously split before the Jewish people, just like the Reed Sea did when they left Egypt. But unlike the Reed Sea which spilt in half, the Jordan is a running river flowing from the north - through the Kinneret, to the south - the Dead Sea. So in the case of the Jordan, the water rose heavenward, over the heads of the Jewish People, for the waters continued to flow down as the Jews passed though the river.
It was at this point, in the middle of the Jordan, with the water rising ever higher and higher, that Joshua stopped the people and told them: "I have an important message to tell you."
The people looked at one another in bewilderment, saying: "Joshua, shouldn't we cross over first to the safety of the other side before you speak to us? What could be so important that cannot wait for a few more minutes until we cross over?"
"No", cried Joshua, "We must talk now!"
One cannot imagine what could have been so important that Joshua had to tell them at that dangerous point and time, that just could not wait. Would it be: Know that you are entering the Land of Israel and you must keep the holy Torah? Would it be: Now that you are entering the Land, I want to set up learning centers? Could it be: We must keep the Shabbat day and make it holy? Was it the importance of eating kosher food?
Interestingly, it was none of the above important concepts of Judaism. Joshua said the following to them: "Know why you are entering the Land of Israel: It is to conquer and remove the nations that live in the Land. If you do so - fine, and if not, then the water of the Jordan will come and wash over you and me together!"
Yes, this was the most important message that Joshua just had to tell the Jewish people at this time, the mitzvah of warfare: We must remove the nations before us, and if not, we will not be able to survive in the Land.
So important is this obligation to fight for the Land of Israel, that we also find in our parsha that when the tribes of Reuven and Gad come before Moshe to ask permission to settle on the eastern side of the Jordan instead of the western side, Moshe makes it a condition that only if they send out their soldiers to fight alongside their brothers in the wars of the Land of Israel, could they then settle on the eastern side of Israel. Moses did not say: You sit and learn in yeshivot while your brothers conquer the Land. Rather, Moses said: Fight with them and then the Land will be yours.
The Rambam lists three different types of obligatory wars, in which all are required to go out and do battle: 1) fighting against the arch-enemy Amalek; 2) fighting the seven nations that lived in the Land of Israel; and 3) troubles which befall the Jewish people, as he explains, when the nations come to wage war against us.
Certainly all agree that the wars that Israel fights today are in this category and are obligatory wars, in which we are commanded to go out to war.
The Mishnah teaches us that in an obligatory war, even the groom and bride leave the wedding canopy to go to war; the men make war and the women help to build roads and prepare food (Tiferet Yisrael)
Among the warriors of King David was Benayahu ben Yehoyada ben Esh Chai, who would later be the head of the army in the time of King Solomon. The Radak (Rabbi David Kimchi) in his commentary tells us "that in spite of the fact that Benayahu was a Kohen and was therefore forbidden to become unclean, still, he would defile himself for the sake of the obligatory wars, just as Pinchas went out to war against Midyan - for in an obligatory war, all go forth!"
It is clear that these days, we must unite as a people, and there is no better vessel in which to do this than the Jewish army of today!