It was necessary because Prohibition was a nationwide ban - until that time the several states reserved the right to make decisions about alcohol availability.
The several states could ban alcohol if they wanted to, just as the states today can ban crack if they want to.
As it stands the Federal government has no power to prohibit drugs unless they cross state borders or the national borders. But the individual states certainly retain that right under the Tenth Amendment to internally legislate.
That seems sensible. The only problem is that it does not conform to reality. The ban on some drugs is also a nationwide prohibition, and, as is evidenced by the Federal interference in medical-marijuana laws passed by several states, the states apparently do not, according to the current regime, have the right to choose whether or not to ban the drugs in question. All the states have the right to do, apparently, is to make state drug laws more harsh than federal laws.
So do you feel that the Federal government is out of place in telling the states what to do on this issue? Do you then agree that a federal Drug Czar is a position for which there is no Constitutional justification?
Bravo! This needs to be said more often; disturbingly, some alleged "conservatives" don't agree.
I found this -- it may be of interest to you. The following is an excerpt:
"An amendment to the Constitution obviously appealed to temperance reformers more than a federal statute banning liquor. A simple congressional majority could adopt a statute but, with the shift of a relatively few votes, could likewise topple one. Drys feared that an ordinary law would be in constant danger of being overturned owing to pressure from liquor industry interests or the growing population of liquor-using immigrants. A constitutional amendment, on the other hand, though more difficult to achieve, would be impervious to change. Their reform would not only have been adopted, the Anti-Saloon League reasoned, but would be protected from future human weakness and backsliding."
There were a few federal statutes passed which dealt with alcohol on a national level prior to Prohibition. To wit (from the same source):
-- The Lever Food and Fuel Control Act of August 1917 banned the production of distilled spirits for the duration of the war.
-- The War Prohibition Act of November 1918 forbade the manufacture and sale of all intoxicating beverages of more than 2.75 percent alcohol content, beer and wine as well as hard liquor, until demobilization was completed.
-- (C)ongressional passage early in 1913 of the Webb-Kenyon Act, a long-sought federal statute against transporting liquor into states that wished to block its entry.
Lastly, one other point to consider. The 21st amendment not only repealed the 18th (in Section 1), it constitutionally moved the legalization decision from the federal government to the states (in Section 2).
Certainly, Section 2 would not be necessary if the federal government had no power over alcohol at the state level -- The Webb-Kenyon Act, coupled with the existing state power to ban alcohol would invalidate the need for Section 2.
An amendment, similar to Section 2 of the 21st amendment, would be necessary to move the drug legalization decision from the federal government to the states. This has the added advantage of allowing each state a voice in this decision, as some states would choose to keep drugs illegal and their concern would be the drug availability in adjacent states. If only 3 or 4 states wished to legalize drugs, the amendment would (rightly) fail. (Unlike alcohol, where all states agreed to legalize -- imagine if only 3 or 4 states chose to do so.)
As it stands now, the Federal gov't can prohibit drugs all it wants since drugs that don't cross state borders are indistiguisable from those that do.