One of the most fundamental bitachon historical incidents happened on Pesach. The king of Ashur a.k.a. Assyria, was conquering much of the inhabited world at the time, and he came upon Yerushalayim as well to wage battle and take control.
At first the Assyrian king sent a message to Chizkiyahu HaMelech to give him a chance to subjugate himself to the vast Assyrian army. The messenger spoke in a belittling manner about the security of Yerushalyim and about having faith in Hashem.
He addressed the Yidden telling them that none of the gods of the other nations saved them thus far and that the Assyrians were victorious in all of their battles. Surely the same will happen to the Yidden, heaven forbid, if they don't devote themselves completely to the king of Assyria.
When Chizkiyahu heard about these announcements he tore his garments and donned sackcloth. He sent a message to Yeshayahu HaNavi asking for guidance in light of these most recent developments.
Yeshayahu gave an encouraging prophecy, that the Yidden had nothing to fear, and all would end in peaceful and quiet manner.
When the king of Ashur heard this response he sent a message to Chizkiyahu, "Do not let your G-d in whom you trust, persuade you that you will be saved. We have already destroyed many lands, and their gods were of no avail to them."
Again Chizkiyahu sent the Assyrian letters to the prophet, and Yeshayahu prophesied a beautiful nevuah in which Hashem promises that not one arrow will be shot at Yerushalayim. Additionally, the king of Ashur is scolded for taking the credit for his victories for himself when it was only because of Hashem's decision that he achieved those victories.
Having received such a clear prophecy, king Chizkiyahu was completely relaxed with no worry of the outcome, in spite of the success that the army of Ashur had enjoyed until that point. In fact the Medrash reports that Chizkiyahu told Hashem that he was totally relying on Hashem to fight the war.
The outcome is known; Hashem sent an angel who wiped out almost the entire Assyrian camp. As another Medrash puts it, this all happened on the night of Pesach.
From the beginning, the message of the Assyrian general was against faith in Hashem. But Chizkiyahu didn't give in and strengthened the Yidden's bitachon in Hashem, that Hashem will be with them and save them from the vast conquering Assyrian army. The Novi also pointed out that the past victories of the Assyrians was only because Hashem gave it to them, and not because of their own strength. The same One who gave them their victories was now going to hinder their battle against the Yidden.
King Chizkiyahu knew that only Hashem can really defeat the enemy, so he placed complete bitachon in Hashem and said, "I sleep with no worries at all as Hashem wages war against them." And that's exactly what happened. While all the Yidden were sleeping at night, Hashem decimated the Assyrian army, in a way that only Hashem can.
We should also benefit from this story, to practice complete reliance on Hashem, not having any worries about the outcome, because when we truly rely on Hashem, the results will be positive in the greatest measure.
 Navi Melachim 2, Perek 18 and 19, and navi Yeshaya perek 36 and 37.
 Aicha Rabosi 4,16.
 Shmos Raba 18,5.