Why post-Civil War America could not annex Japan

What would Britain do if the United States tried to annex Japan after the Civil War?

Britain would likely obstruct the attempt or arm Japan, and the United States lacked the logistics and force structure to carry it out.

Post-Civil War America did not possess its Second World War scale of power projection. Reconstruction, internal instability, limited infrastructure, and an inadequate force structure made a trans-Pacific invasion and occupation unrealistic. Britain would probably oppose so large an American gain, either by blocking it directly or by selling weapons to Japan. A declaration of annexation would run far ahead of the United States' ability to enforce it.