A correspondant asked:
You said the Jews are still obligated to keep the Law. How are they supposed to do that when they don’t know what tribes they are from and don’t have the things they need to do it because God Himself destroyed everything?
The main tribal requisites that are were pertinent to keeping the Law had to do with the tribes of Levi and Judah. God promised that the Messiah would rise from Judah, and He promised that Levi would not cease being a tribe (Jer 33:21-22). The Jews do in fact know who the descendants of Levi are today, even as Jeremiah said they would. Many of them have the name Cohen, which is a variant of the Hebrew word kohonen, which means “priest”. These Cohens have a strong family similarity in their DNA. However, other than the priests, there were no tribal requirements in order for Israel to obey the Law, and the priests are known.
You are right that God destroyed them, but unlike us, what He destroys, He is able to restore, and He promises to do so. Here is what II Baruch 6:5-10 says of the things of the temple that God destroyed during the siege at carrying away into Babylon:
II Baruch 6:5-10
5 And another angel began to descend from heaven. and said unto them: ‘Hold your lamps, and do not light them till I tell you. 6 For I am first sent to speak a word to the earth, and to place in it what the Lord the Most High has commanded me.’ 7 And I saw him descend into the Holy of Holies, and take from there the veil, and holy ark, and the mercy-seat, and the two tables, and the holy raiment of the priests, and the altar of incense, and the forty-eight precious stones, wherewith the priest was adorned and all the holy vessels of the tabernacle. 8 And he spoke to the earth with a loud voice:
‘Earth, earth, earth, hear the word of the mighty God,
And receive what I commit to you,
And guard them until the last times,
So that, when you are ordered, you may restore them,
So that strangers may not get possession of them.
9 For the time comes when Jerusalem also will be delivered for a time,
Until it is said, that it is again restored for ever.’
10 And the earth opened its mouth and swallowed them up.
There are prophecies that require that the 10 tribes also return and be reunited with Judah (the Jews). I do not know how God has maintained these Ten Tribes, but He says that He will do so, and I believe Him. Hosea 1:11 says, “Then shall the children of Judah and the children of Israel be gathered together, and appoint themselves one head, and they shall come up out of the land: for great [shall be] the day of Jezreel.” Also, Ezekiel 37 requires that Israel and Judah be reunited. This general reunion appears to happen very close to the end of the world, for Hosea says that once the two nations are reunited, they select a common head, and “come up out of the land”! The only time Israel will leave the land of their inheritance is to go to the Sheep Pen at Bozrah to await the Lord’s return (Micah 2:12-13).
Here is a non-canonical account of the time of the return of the Ten Tribes as found in
II Esdras 13:40-47
40] Those are the ten tribes, which were carried away prisoners out of their own land in the time of Osea the king, whom Salmanasar the king of Assyria led away captive, and he carried them over the waters, and so came they into another land.
 But they took this counsel among themselves, that they would leave the multitude of the heathen, and go forth into a further country, where never mankind dwelt,
 That they might there keep their statutes, which they never kept in their own land.
 And they entered into Euphrates by the narrow places of the river.
 For the most High then shewed signs for them, and held still the flood, till they were passed over.
 For through that country there was a great way to go, namely, of a year and a half: and the same region is called Arsareth.
 Then dwelt they there until the latter time; and now when they shall begin to come,
 The Highest shall stay the springs of the stream again, that they may go through: therefore sawest thou the multitude with peace.
So a multitude of sources testify that all Israel will be united before the end of the world and her fortunes will be restored. We can either believe it or not, but God says it.