Why the Union still needed overseas trade in 1861

Could the Union simply replace lost foreign trade with domestic industry?

Eventually it could expand domestic output, but in 1861-62 imported materials and munitions remained critical.

Domestic production could expand, but not instantly. Saltpetre for explosives, finished guns, shells, and other inputs still came from abroad in 1861-62. Building substitute capacity required time, capital, and workers at the same moment the Union was mobilizing enormous armies. A British disruption of trade would therefore bite during the vulnerable transition rather than against the mature industrial system of 1865. The strategic danger lies in having to solve the production shortfall while simultaneously fighting the Confederacy, defending Canada-facing borders, and building a larger ocean navy.