A common theme in the Old Testament is that Israel's safety and salvation rest in God, not in military power or foreign alliances. God does not delight in the strength of horses, but in those who hope in God's steadfast love (Psalm 147:10-11); similarly, faithful Israel takes pride in God, not in horses and chariots (Psalm 20:7).
But Israel frequently forgets or refuses to rely on God's promise. When they do, they bring judgment upon themselves, as in this text. The horses they wanted for war will now serve only for their retreat before the enemy (see Isaiah 31:1-3; Psalm 33:17).
In Isaiah's time, the prophet warned against an alliance with Assyria, observing that it would only bring disaster (see Background)--and Isaiah's word proved correct.
Throughout the Bible, true strength is found in weakness rather than in human power (1 Corinthians 1:25; 2 Corinthians 12:9)--and this has always been a difficult lesson for God's people to accept. The biblical promise of strength in weakness may or may not have anything to do with the foreign policy of a particular nation once God's people no longer live as a defined nation-state as they did in the Old Testament. Still, the recognition that God calls for faithful reliance on God's promises rather than self-help salvation will always be a significant message for believers to keep alive in all human affairs.
15 For thus said the Lord God, the Holy One of Israel:
In returning and rest you shall be saved;
in quietness and in trust shall be your strength.
But you refused 16and said,
No! We will flee upon horses
therefore you shall flee!
and, We will ride upon swift steeds
therefore your pursuers shall be swift!
17 A thousand shall flee at the threat of one,
at the threat of five you shall flee,
until you are left
like a flagstaff on the top of a mountain,
like a signal on a hill.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org