In April 1804, Pitt was gathering his forces against the current, ineffectual government. The war with France was proceeding far from well, and Henry Addington had a rapidly diminishing parliamentary majority. He was simply no match for Pitt's superior oratory and the sheer logic of his persuasive arguments. Although a personal friend of Addington, Pitt had become increasingly critical of Addington's defence policies since he became premier in 1801. By April 1804, Addington's government was teetering on a knife edge. Pitt hoped that a combined Coalition government, with the inclusion of the Foxites and the Grenvillities, would prove a strong enough force to finally defeat the dictator, Napoleon Bonaparte.
On April 11, 1804, Pitt was staying at Walmer Castle, his headquarters as Lord Warden of the Cinque Ports, and planning his next move. He addressed an unknown political correspondent, asking for his support, as follows:
My dear Sir
I have had no Opportunity lately of knowing what may be your Sentiments with respect to Public Affairs; but seeing that they are likely very soon to lead to an important Crisis, I am sure from our long habits of acting together and from the many Proofs I have received of your Friendship and Good Opinion, you will readily excuse my troubling you with this Confidential Letter. You will have seen from what has passed in Parliament how much I have been dissatisfied for a considerable Time with many parts of the Conduct of Government, particularly in the essential Article of what relates to the defence of the Country. The Experience of the last Summer and the discussions of this Session confirm me in the Opinion that while the Government remains in its present Shape and under its present Leader, that nothing efficient can be expected either to originate with them or to be fairly adopted and effectually executed.
With this persuasion and thinking that a System of more Energy and decision is indispensable with a view both to the immediate Crisis, and the many difficulties We may have to encounter in the Course of the present Contest. I mean to take an early Opportunity of avowing and acting on these Sentiments more explicitly and decidedly than I have hitherto done; and I shall endeavor to give Effect to my Opinion by the Support of all the Friends whom I can collect. - My Object will be to press to the utmost those Points which I think essential to the Public defence, and at the same time in doing so to make it if I can impossible for the present Government to maintain itself. In this Object I have every reason to believe that I shall have the fullest Concurrence of all those with whom I have the most differed on former Occasions, and with whom possibly I may as little agree in future. With their Numbers added to my own more immediate Friends, and to the few who have acted with Ld. Grenville and Windham, I am persuaded that our division on any favorable Question will probably be met as would be sufficient to make a much stronger Government than the present, and if a considerable Strength shews itself in Parliament, I have no reason to suppose that any insuperable difficulties will arise in another Quarter. Calculations of Strength beforehand are necessarily uncertain, but I think at lowest our Numbers cannot be much less than 200, and I should not be surprised if they were considerably more. - I have thus taken the Liberty of explaining to you very frankly my Views and Expectations. I do not feel that I have any Claim to ack[nowledge?] your Concurrence and Support; or to do more than to lay the Subject fairly before you. If your own View of the Subject coincides with mine, it will certainly be highly gratifying to myself personally, and your Weight and Influence with your Friends will I have no doubt in that case secure in a valuable Accession of Strength. I am, with great regard, My dear Sir, Faithfully & Sincerely Yours, W. Pitt.” 
By the end of April 1804, Addington had lost his parliamentary majority, and was forced to resign. On May 10, 1804, Pitt returned to the premiership for the second (and final) time. Unfortunately, the King disapproved of the inclusion of Fox, and with the desertion of the Addingtonians (the supporters of Addington), as well as the Grenvillities, Pitt's second administration was considerably weaker than he had intended. One wonders if the war would have proceeded differently (i.e. with more effectiveness) had stubbornness and political rivalry not gotten in the way of taking a united stand against Napoleon.
1. William Pitt to 'My dear Sir' (unknown recipient). British Library Add Ms 37538, ff. 45-46.