Austria was in action, and England, France, and Spain were not idle either. First, the Spaniards seized Haiti Island, followed by the three countries accelerating their expansion in Mexico.
The British had just quelled the Indian rebellion, and the French were still focused on the Italian Area. So this time, Mexico was luckier than in history, as the countries' expansions were mainly through economic and political infiltration.
Had they not been wary of aggravating the two American governments in the midst of civil war, they might have even attempted to infiltrate the American states.
Re-establishing a colonial system in North America has always been the dream of English, French, and Spanish colonists. The Monroe Doctrine of the Americans blocked everyone's path, and now naturally, they wanted to kick away this obstacle.
London
As the world's foremost naval power, the British rightly became the leader in the countries' joint intervention plan in the United States.