Surely the Egyptians are vanity. [1] This verse contains the explanation of the former statement; for he repeats and threatens the same thing, that the Egyptians, after having worn out the Jews by various annoyances and by prodigious expense, will be of no service to them. "The strength of Egypt" will do them no good, even though he be earnest in assisting them, and employ all his forces. Thus shall the Jews be disappointed of their hope, and deceive themselves to their great vexation. The particle v (u) signifies here either for or surely, as I have translated it. Therefore have I cried to her. He now shews that the Jews have no excuse for fleeing with such haste into Egypt, and that they are willingly foolish and unworthy of any pardon, because they do not repent when they are warned. When he says that he "cried" to Jerusalem, I consider this to refer to God himself, who complains that his distinct warnings and instructions produced no effect, and that his exhortation to them to sit still was not without foundation, but was intended to meet the troubles and calamities which he foresaw. Whence came that restlessness, but because they refused to believe the words of the Lord? In a word, he shews that it is mere obstinacy that drives them to flee into Egypt; for by "sitting still" they might provide for their safety. By the word "cry" he means that he not only warned them by words, but likewise chastised them; and this makes it evident that their obstinacy and rebellion were greater. "To sit still" means here "to remain and to stay at home," though he will afterwards shew (verse 15) that they ought to have peaceable dispositions. The cause of their alarm and impassioned exertions was, that they were terrified and struck with dismay, and did not think that God's protection was sufficient, if they had not also the Egyptians on their side. Thus, they who do not give sufficient honor to God have their hearts agitated by unbelief, so that they tremble and never find peace.

Footnotes

1 - "For the Egyptians shall help in vain." -- Eng. Ver.

For the Egyptians shall help in vain - That is, if they enter into the alliance, they shall not be able to defend you from the invader. The other member of the sentence would seem to imply that they would make promises of aid, and would even boast of being able to deliver them, but that they would fail in their promises.
Therefore have I cried - Therefore have I the prophet cried, that is, I do call her so.
Concerning this - Concerning this country; that is, Egypt. Some have understood this as referring to Jerusalem, but the connection requires us to understand it of Egypt.
Their strength is to sit still - This is evidently designed to be an expressive appellation of Egypt. The word rendered here, without much propriety, 'strength' (רהב râhab) is a proper name of Egypt, and is several times applied to it; Isaiah 51:9 :
Art thou not it that hath cut Rahab
And wounded the dragon?
In this passage there can be no doubt that it refers to Egypt. So in Psalm 87:4; Psalm 89:10 (see the margin). Why it was given to Egypt is unknown, and can only be conjectured. Bochart ("Geog. Sacra," i. 4. 24) supposes that it is derived from the word ῥιβι ribi, which singifies "a pear," and that it was given to the Delta or Lower Egypt on account of its form, as somewhat resembling a pear. But there is not clear evidence that such was the meaning of the word, and there is no reason why we should forsake the usual sense of the Hebrew word. The verb רהב râhab means to urge, press on, attack Proverbs 6:3; to be highspirited, fierce, full of courage; to behave proudly Isaiah 3:5; and has, in most instances, a relation to pride, to arrogance, to boasting Job 9:13; Psalm 40:4. The noun "Rahab" indicates ferocity, haughtiness, boasting, insolence; and the name was doubtless given to Egypt on account of its insolence and pride. It is used here because Egypt would be full of self-confidence, and would boast that she could aid the suppliant Jews, and deliver them from the threatened invasion. The phrase rendered 'to sit still,' is a part of the name which the prophet gave to her. Though she boasted, yet would she sit still; she would be inefficient, and would do nothing; and the whole name, therefore, may be rendered, 'I call her, the blusterer that sitteth still;' that is, 'they are courageous in talking; cowards in acting.' (Taylor)

Their strength is to sit still "Rahab the Inactive" - The two last words, הם שבת hem shabeth, joined into one, make the participle pihel המשבת hammeshabbeth. I find the learned Professor Doederlein, in his version of Isaiah, and note on this place, has given the same conjecture; which he speaks of as having been formerly published by him. A concurrence of different persons in the same conjecture adds to it a greater degree of probability.

For the Egyptians shall help in vain, and to no purpose: therefore have I cried (e) concerning this, Their strength (f) [is] to sit still.
(e) That is, to Jerusalem.
(f) And not to come to and fro to seek help.

For the Egyptians shall help in vain, and to no purpose,.... Not sending help in time, or such as did no service; though they made a show of help, and attempted to help them, or seemed to do so, yet failed to do it:
therefore have I cried; proclaimed or published, either the Lord by the prophet, or the prophet in the name of the Lord, which is much the same:
concerning this, Their strength is to sit still; either concerning this embassy, that it would have been better for the ambassadors to have spared all their toil, and labour, and strength, in going down to Egypt, and have remained quiet and easy in their own country: or, "I cried, or called, to this (i)", this city of Jerusalem, and the inhabitants of it, and declared to them, that it was best for them quietly to trust in the Lord, and depend upon his protection, and sit still in Jerusalem, and not attempt to flee from thence to Egypt for safety, and they should see the salvation of God, as in Exodus 14:13 to which some think there is an allusion; not but that they might be busy, and employ themselves in preparing for their defence, by providing themselves with arms, and repairing their fortification; but it was not right to go out of the city, and seek a foreign aid or safety. The word for "strength" is "Rahab", one of the names of Egypt, Psalm 87:4 and so the sense may be, their "Rahab", their "Egypt", or what they expect from thence, namely, protection and safety, is to sit still, and abide quietly at Jerusalem. Jarchi refers this to Egypt, "I have called to this", to Egypt, they are of a proud spirit, the people cease, and are proud without cause; or according to another exposition he gives, their pride ceaseth, or it is fit it should. De Dieu interprets it also of Egypt; and so does Gussetius (k), but in a different manner, thus, the Egyptians are strength as to rest, they will strongly rest, while Israel strongly hopes they will help them.
(i) "vocavi ad hanc", Montanus; "ad istam clamo", Castalio. (k) Comment. Ebr. p. 829.

"Egypt is vanity, and to no purpose will they help" [G. V. SMITH].
strength--Hebrew, Rabah, a designation for Egypt (Isaiah 51:9; Psalm 87:4), implying her haughty fierceness; translate, "Therefore I call her Arrogance that sitteth still." She who boasted of the help she would give, when it came to the test, sat still (Isaiah 36:6). English Version agrees with Isaiah 30:15; Isaiah 7:4.

To her - To Jerusalem or Judah. Sit still - It is safer and better for them to sit quietly at home, seeking to me for help.