To understand this riddle you have to differentiate between!:
- "the house of Israel" - who the riddle is directed/addressed to (Israelites!) - see Exekiel 17:2
- "the remnant of the house of Judah" - who the riddle is about (Jews!)
The 'house of Israel' (the 10 tribes which came to make up the northern kingdom of Israel) was taken into Assyrian captivity many years prior to the events portrayed in this "parable". - see 2 Kings 18:9-11
The 'house of Judah' (the 3 tribes which came to make up the southern kingdom (more specifically, Jerusalem)) at the time of this riddle had recently been subjugated by the king of Babylon. And the king in Jerusalem is reaching out to the king of Egypt for military assistance to regain their sovereignty.
Since God has arranged for the Babylonian captivity of the residue of the house of Judah, He is very displeased with Jerusalem's attempt to undercut His will/plan and is going to diminish/overturn Jerusalem and to strengthen/bring back the house of Israel to prominence.
Ezekiel 17 22Thus saith the Lord GOD; I will also take of the highest branch1 of the high cedar2, and will set it; I will crop off from the top of his young twigs3 a tender one4, and will plant it upon an high mountain5 and eminent: 23In the mountain of the height of Israel6 will I plant it: and it shall bring forth boughs, and bear fruit, and be a goodly cedar: and under it shall dwell all fowl of every wing7; in the shadow of the branches thereof shall they dwell. 24And all the trees8 of the field9 shall know that I the LORD have brought down the high tree10, have exalted the low tree11, have dried up the green tree12, and have made the dry tree13 to flourish: I the LORD have spoken and have done it.
1highest branch - symbolic of the king in Jerusalem (of the line of David?)
2high cedar - symbolic of Jerusalem (residue of the house of Judah)
3top of his young twigs - symbolic of royal family
4tender one - symbolic of young heirs (daughter(s)?) of the king
5high mountain - symbolic of a prominent nation/peoples
6Israel - synonymous with house of Israel (who the riddle is being "put forth" to)
7all fowl of every wing - symbolic of those from every kingdoms/nations/peoples
8trees - symbolic of nations/peoples
9field - symbolic of the world
10high tree - symbolic of house of Judah
11low tree - symbolic of house of Israel
12green tree - symbolic of house of Judah
13dry tree -symbolic of house of Israel
It would appear that God is transferring the remaining heir(s) of the line of kings in Jerusalem (the line of David?) to transplant them into the house of Israel wherever they are located at that time.