These were the future generations, the ones that did not know how to appreciate the miralces that were done for Joshua at the time of the war with Canaan. Because of this, their faith in God weakened, and that is why God was able to test Israel by means of the enemy nations.
(ב) ללמדם מלחמה. רצה לומר, הנה בהשאיר הגוים, הלא ילחמו בם ויהיה אם כן מהצורך ללמדם טכסיסי מלחמה, כי רק הדור אשר היו לפנים בימי יהושע, המה לא ידעו טכסיסי מלחמה, כי ה׳ נלחם להם, אבל לאחר זה יצטרכו לדעתם, כי בעבור מעשיהם יעזבם ה׳ אל המקרה, ואז ישכילו לדעת הנסים הנעשים מאז ויאמינו מעתה בה׳:
To make them experience (teach) them war. In other words, with the other nations aroudn them, would they not have to war against them? And if that's the case, it would be necessary to teach them military techniques. The generation before them in the days of Joshua did not know warcraft because God did the fighting for them. But afterwards, the Israelites had to know how to wage war. On account of their sins, God had abandoned them to random chance so that they would come to appreciate the miracles that were done in the past and so they would believe in God.
The test was in two parts. Part one was learning how to wage war. Part two was to see if they would listen to God's commandents and foreswear idolatry and intermarriage.
The Israelites did not pass these two tests. Their first failure was that they learned war but did not fight; they lived with the Canaanites and made treaties with them rather than fighting them. Their second failure is that they intermarried.
Up until this point is the general introduction to all of the judges in the Book of Judges. It describes what was done from the death of Joshua through the whole era of the Judges. Now the text goes into detail, what was done in the days of each individual judge. It says that Israel began to do what was evil in the eyes of God, so much so that they forgot God and worshipped the Baalim.
..."And God strengthened Eglon..." This means that Eglon was a weakling, and that under normal circumstances, Israel would have conquered him. But the Israelites committed evil acts, God strengthened Eglon so that he defeated Israel...Also their evil acts incited their neighbors to attack them, as their enemies thought, "Their God has abandoned them..."
A left handed man: The reason that the king of Moab was not able to defend himself from Ehud's dagger is that Ehud was lefthanded. Therefore, he holstered his dagger on his right so that it would be convenient to seize it with his left hand. Otherwise, the king would have seen something sticking out on Ehud's left side and he would have suspected the presence of a weapon...
A stout man: This is another reason for Eglon's death, that he was a fleshy, obese man, and it was very hard for him to get up, and he wasn't able to defend himself from Ehud.
He attacked while Eglon was still having trouble standing up.
The fat closed over the blade: So that Eglon would die immediately and not cry out, he thrust the blade deeply. Ehud did not pull out the blade because he was afraid that people would know what he had done if he came out with blood stains on him.
And the filth came out: The smell of the filth caused his guards to think that the smell was from the king going to the bathroom.