Why on earth would the UK start a war with Spain?
If Spain attacked and invaded Gibraltar, Spain would cause a global financial crisis let alone an EU meltdown.
Spain is doing the same thing Sturgeon is doing, using Brexit as a political opportunity to exert pressure on the UK to hand over power.
This may be in the Spanish interests since it has long coveted Gibraltar, but if it tried to veto the EU brexit negotiations, all parties in the EU (including Spain) and the UK would lose.
By threatening to do so, Spain is placing her own national interests above that of the EU or any other country within the EU.
The Spanish position is in nobody's best interest.