Research on militia and National Guard
I've been doing some research on the legal origins of the two, and found some tidbits.
The present federal definition of militia (10 USC 311) as including all able-bodied males 18-45, and women in the Guard, divided into the Organized Militia and the Unorganized Militia, originates in the Dick Act of 1903 (32 Stat. 77): "the militia shall consist of every able-bodied male citizen .... who is more than 18 and less than 45 years of age, and shall be divided into two classes--the organized militia, to be known as the National Guard of the state ... and the remainder, to be known as the Reserve Militia." (Note effect on the NG=militia argument: the NG is only part of the militia). Certain federal benefits were granted, but only to NG units.
The 1903 Act retained the power of the President to call out the militia (including the unorganized) in event of invasion, insurrection, or to execute the laws of the Union (the three purposes for which it could be called out under the Constitution).
Then came foreign troubles -- first, the raids of Pancho Villa. In 1912, the Attorney General ruled that the militia (including the Guard) could not be sent outside the US. The three constitutional allowances for calling up the militia had been selected to include only things do-able inside the US. So how do you use the Guard for foreign duty?
The 1916 Act, 34 Stat. 166, 210-11, provided for simply drafting the National Guardsmen, and provided that upon being drafted their service as militia were terminated. That was done in WWI, to the great discontent of the Guard, since units were broken up. (Note the effect on NG=militia: in this case, the moment the Guard were called up, they ceased to be even part of the militia). The 1916 Act also created the concept of "federal recognition" of Guardsmen; to be eligible for federal benefits a Guardsman had to be accepted by federal authorities. (The 1920 Act, 41 Stat. 759, 784, fixed a problem created--namely, the NG ceased to be state militia when called up, which meant that when the war was over they were done with their NG duties even if their NG enlistment had not run out. The 1920 Act provided that States could require that they finish their enlistment). Note again the effect on NG=militia; from 1916-1920 Congress asserted the power to simply, and permanently, remove Guardsmen from state militia status.
Then came the 1933 Act, 48 Stat. 153, 155. This provided that NG officers were to be commissioned in the Army of the US, and that the NG of the US "shall be a reserve component of the Army of the United States...." Officers of the Guard were to be appointed by the President (the Constitution says appointment of militia officers is reserved to the States). It allowed NG officers to be terminated by a board appointed from the Army or NG as the Secretary of War might direct. The Militia Bureau of the Dept of War was renamed the National Guard Bureau. Drafting of Guardsmen was replaced by a Presidential power to call up Guard units, which would then cease to be part of the NG of their state.
In short: pre-1903, National Guard had no particular Federal legal status as such. It was a title of certain units (which became popular title for organized militia after Lafayette's visit to the US in the 1820s, he having organized units by the name in France). 1903, it is recognized, but only as part of the militia of the US. 1933, it really becomes part of the Army.