The council was suspended, not concluded. In July 1870 the Franco-Prussian war began and the French garrison protecting Rome withdrew. In September Italian troops entered the city and the Papal States ended. The bishops dispersed, the council was suspended in October, and its unfinished agenda, which included a full treatment of the church of which the section on the papacy was only one part, was never taken up. Vatican I was formally closed only in 1960, by John XXIII, as he prepared to call another council.
The consequence of that interruption is a doctrinal imbalance that shaped Catholic life for ninety years. The council defined the authority of the pope and had no opportunity to define the place of bishops, of local churches or of the laity beside it. What was intended as one chapter stood alone as though it were the whole.
Vatican II is best read as the completion of that unfinished business. Its constitution on the church sets episcopal collegiality beside papal primacy, teaches that bishops succeed the apostles as a body, and describes the church as the whole people of God before it describes its hierarchy. Neither council reversed the other. The second supplied the context the first had been prevented from providing, which is why Catholic theologians routinely insist the two must be read together.