O Lord - This is a solemn prayer to Yahweh, made by the Jews in the apprehension of the invasion of the Assyrian. It is not meant that this prayer was actually offered, but it is a prophetic representation indicating the alarm of the Jews at his approach, and their disposition to throw themselves upon the mercy of God.
We have waited for thee - That is, we have looked for deliverance from this threatened invasion from thy hand (compare the note at Isaiah 26:8).
Be thou their arm - The arm is a symbol of strengh. It is used in the Scriptures as emblematic of the divine protection, or of the interposition of God in time of calamity and dancer Exodus 15:16; Job 40:9; Psalms 44:3; Psalms 77:15; Psalms 89:21; Psalms 98:1. Lowth proposes to read ‘our arm instead of ‘their arm;’ and the connection would seem to demand such a reading. The Vugate and the Chaldee read it in this manner, but there is no authority from manuscripts for a change in the text. The truth seems to be, that Isaiah, impelled by prophetic inspiration, here interposes his own feelings as a Jew, and offers his own prayer that God would be the strength of the nation. The form, however, is immediately changed, and he presents the prayer of the people.
Every morning - Constantly; at all times.
In the time of trouble - Referring particularly to the trouble consequent on the invasion of the Assyrians.