They are already allowed to fight against foreign soldiers. Heck, that's their job.
If Liberals would agree, as you state, that foriegn nationals plotting to commit acts of terror are "soldiers", and therefore legitimate targets for our military, I would be comfortable with the implication of your statement. Unfortunately, Liberals talk of due process for these foriegn nationals and don't categorize them as soldiers.
For instance, if a national gaurdsman had shot the Arab terrorist at LAX two weeks ago rather than El Al security, there would be a furor in the press about the unlawful nature of the militaries action. One wonders whether the gaurdsman would have been charged.