A version of this is well known as the slogan of the US Armed Forces: the difficult we do immediately; the impossible takes a little longer. The ‘French Minister’ to whom this saying is attributed in quot. 1873 is Charles Alexandre de Calonne (1734-1802), appointed finance minister by Louis XVI in 1783: si cʾest possible, cʾest fait; impossible? cela se fera, if a thing is possible, consider it done; the impossible? that will be done (quoted in J. Michelet Histoire de la Révolution Française (1847) i. ii. 8).
What was it the French Minister said. If it is simply difficult it is done. If it is impossible, it shall be done.
[1873 Trollope Phineas Redux II. xxix.]
The impossible may take a while, but we do it, you know the routine.
[1967 H. Harrison Technicolor Time Machine iv.]
A well-worn precept of the British Navy was that the difficult was done at once; the impossible took a little longer.
[1981 P. Mccutchan Shard calls Tune iv.]
That's good, utilitarian, achievement-oriented American lingo. We do the difficult immediately, the impossible takes a little longer.
[1997 National Review 29 Sept. 66]
Related to: possibility and impossibility
Bibliography of major proverb collections and works cited from modern editions is available here.
