Why didn't Britain send troops ASAP to Portugal to aid in the expulsion of French forces?
I personally suspect that the British government just didn't see it as militarily viable to do so, with Spain still being a reluctant ally of France. Even if the British and Portuguese could expel the Spanish and French temporarily, they could easily reinvade with twice the numbers and drive the British into the sea with nothing being gained and time, men and resources being wasted.
I also suspect that the British only saw liberating Portugal as being potentially viable, when the alliance between Spain and France ends (because then there is something of a bulwark between the two nations).
Does anyone have a definite explanation of this?
Most important is that the British Government informed the Portuguese Government at the time, that given the large military involvement in Denmark and the Baltic straits, and also due to their important troop deployments in Sardinia and Sicily, Britain was not able to send troops to help Portugal stopping a Franco-Spanish invasion.
Anyway, London did not believe that Portugal was defensible against a French and Spanish invasion. That is why Portugal was pressed to move the Royal Family and Government to Brasil, together with the important Portuguese Fleet.
Assuming (I am probably wrong) you have some issues with the Portuguese language, please find the following references in English:
Gray, Anthony (2011). The French Invasions of Portugal 1807-1811: rebellion, reaction and resistance. MA thesis. York University.
https://etheses.whiterose.ac.uk/2569/1/MA_Thesis_The_French_Invasions_of_Portugal_1807-1811_Online_Version.pdf
Horward, D. D. (1989). Wellington and the Defence of Portugal. The International History Review, 11(1), 39–54. http://www.jstor.org/stable/40105954
In late 1807 it wasn't militarily viable for all sorts of reasons, the weather at that time of year for ships, the number of French troops in the Peninsula far outweighing what Britain had available etc. Also, the major concern - that of the Portuguese fleet falling into French hands - had been dealt with when it sailed for Brazil. The Russian squadron was safely blockaded in Lisbon harbour and while there were early discussions about an operation to take the Tagus forts (with Spencer's and/or Moore's troops) and destroy the Russian ships a wider invasion wasn't contemplated.
It was only once the Spanish rose up in May that some form of large scale intervention became doable with the numbers of troops available, and even then the primary object was not to liberate Portugal for its own sake (or that of the old alliance), but to support the Spanish by securing the rebel's flank. If the Spanish had asked for troops in the north or at Cadiz then Wellesley would have probably done that, as his orders allowed.
Hi @Daniel Ross , I will leave it to more experienced Peninsular hands to answer in detail. It us to be remembered though that Britain had a professional but small army, incapable of expeditionary campaigning on the continent without considerable care.