"The following song shall be sung on that day in the land of Yehudah: The city of our strength shall be a salvation like a high and low wall." - Isaiah 26:1
The Presence of G-d around Jerusalem's righteous will protect them better than any physical wall. That presence will bar the spiritually unclean and keep out all spiritual enemies. G-d's Presence will be so evident in Jerusalem, that no gentile nation would dare attack it. But, for G-d's Presence to be revealed and become "like physical walls," we must be spiritually strong so that the city's defenses are "our strength."
The first prayer to be said in Jerusalem when the righteous return through its gates will be a prayer for protection from the inner evil - each one's own evil inclination. May it be subjugated to higher goals so that people will not turn to sin. This will bring about several levels of peace: between body and soul, among people, and between Jew and Gentile.
"Trust in G-d forever, for the Lord, G-d, is the Foundation of the worlds." - Isaiah 26:4
Isaiah now urges to strengthen one's faith, not to rely on physical walls to foreign treaties. Firm trust in G-d will open up spiritual channels of help from beyond this physical world. Place trust in G-d "forever" - even in the most dire circumstance. He Who is the Foundation of the worlds can bring salvation from a higher world.
Even when we stray away from the straight narrow path, when G-d metes out justice, we know it is coming from Him and we yearn for Him while enduring His justice. And we do not serve Him just to avoid His justice, but as an expression of our yearning of soul.
"My soul yearns for You during the night, the spirit inside seeks You at dawn. For when Your judgment reaches the earth, the world's inhabitants learn righteousness." - Isaiah 26:9
Speaking both for himself and for the Jewish people at large, Isaiah expresses a deep yearning for G-d. Daily at night and first thing in the morning, Isaiah secluded himself with G-d. Those are the times for deep meditation, when the ways of the world are stilled. Those are the times to see G-d's justice on earth, how all His actions are to make mankind righteous.
"G-d grant us peace! For You have already punished us for all our sins." - Isaiah 26:12
All the blessings that the Future will bring will be worthless without the blessing of peace.
Why are the Jewish people worthy of the blessing of peace? Because despite being in exile under gentile masters who would have given them full rights had they accepted their gentile faith, only You did they call their G-d. Why did they not accept the gentile gods? - because they are dead and have no life. Therefore, G-d will destroy all those who worship them and leave them as dead as their gods.
The Jewish people in exile were persecuted and hounded - they had no land and were a small and weak people, but it was they who brought G-d honor. The mighty nations, however, whom G-d had waxed great and who had vast lands and populations, did they ever give G-d any honor?
Whenever the Jewish people in exile were met with distress, they turned their hearts straight to G-d, realizing that the Source of their distress was G-d. They accepted those pains as a woman does labor pains, knowing that in the end will be birth - Redemption. They accepted those pains knowing that Redemption will only come after such "labor pains." But just as those pains may last a long time, so will the exile last long. And many times there were pains which culminated in stillbirth, without the Redemption arriving, yet just as the birthing mother screams not in complaint, the Jewish people did not scream in complaint. But why has Redemption not come after all, why does Jewish rebirth so tarry? Because just as a birthing mother screams out of pain and not out of regret for its cause, so too the Jewish people scream out of their pain and not out of regret for their sins - the cause.
"Your deceased will live again, My corpses will rise up; rouse and sing, dwellers of dust! For Your dew is the dew of Light and the earth shall give forth the dead." - Isaiah 26:19
"Go, My people, into your rooms and close your doors behind you; hide for a short while until wrath will pass." - Isaiah 26:20
But before that will happen, there be great travails and only those who hide will survive - those who hide in their "rooms" of prayer and study, those who search the deep "rooms" of their hearts.
"For G-d is coming forth from His place to mete out judgment upon the earth's dwellers for their sins. The land will then uncover its blood and not conceal its killed anymore." - Isaiah 26:21
Those travails will entail judgment on those who filled the earth with Jewish blood. G-d will "leave His place" of extended mercy to mete out due punishment on them. Only then will the earth avenge herself of the Jewish blood that she was forced to swallow.