I see that Arizona's Randy Terrill had the same thought I did:

"As of now fourteen states–Arizona Alabama, Delaware, Idaho, Indiana, Michigan, Mississippi, Montana, Nebraska, New Hampshire, Oklahoma, Pennsylvania, Texas and Utah–are on board with this attempt to eventually trigger a Supreme Court review of current federal policy (as opposed to law or court decisions) on what Rep. Kavanagh referred to as granting citizenship 'based on GPS location at birth.'  Randy Terrill,  an Oklahoma Republican state representative working on an anti-birthright bill there,  goes further. 'When taken to its logical extreme, that would produce the absurd result that children of invading armies would be considered citizens of the U.S.,' he said."[Emphasis added][The Next Immigration Battle: Birthright Citizenship, Arnold Ahlert, FrontPage Magazine, January 7, 2011]

Terrill's remark about invading armies is a normal, ordinary thing to think, and it doesn't necessarily mean that he's reading VDARE.com, but I just want to point out that I thought it first, over six years ago:

Consider the following hypothetical case:

Imagine that an invading army enters the United States, wearing uniforms and carrying arms, and behaving exactly like an army, but bringing their wives.

This is what the European armies of the early nineteenth century did, during the Napoleonic Wars, after all. Women frequently gave birth in foreign countries.

If this hypothetical invading army stayed long enough to have children, would the enemy soldiers' sons and daughters be American citizens?

Would those children have the right to return to the US, years later, and sponsor their soldier fathers, who would presumably have been expelled in this hypothetical war?

The idea is ridiculous.

However, it happens today that the illegal immigrant population is larger than an invading army.[The Fulford File, By James Fulford | Children of an Invading Army, June 29, 2004]