Has this already happened since 1948, or is this something that has begun to be fulfilled, but is to be completely fulfilled in the kingdom?

Ezekiel 37:21 And say unto them, Thus saith the Lord GOD; Behold, I will take the children of Israel from among the heathen, whither they be gone, and will gather them on every side, and bring them into their own land:

  22 And I will make them one nation in the land upon the mountains of Israel; and one king shall be king to them all: and they shall be no more two nations, neither shall they be divided into two kingdoms any more at all: