Consider the French decision to declare war on Prussia in 1870. What was the strategic balance (or imbalance) between the two at the time?

The Franco-Prussian War was launched when France was under the rule of Emperor Napoleon III (nephew to Napoleon I) and Prussia was under the political leadership of Bismarck and William I. During this period, both countries were actively trying to expand their political influence, with Prussia the more successful of the two (given that these maneuvers would ultimately result in the creation of a unified Germany).
Louis Napoleon Bonaparte proclaimed himself Emperor of France in 1852, during which time he oversaw a period of extensive economic growth. Meanwhile, he actively sought to expand France's presence in Europe and the world. In 1859, he aligned with Piedmont-Sardinia in waging war against Austria and soon afterwards began imperial adventures in Africa, the Middle East, and Asia. An attempt to extend French influence into Mexico (by supporting would-be Emperor Maximilian) ended in disaster.
Meanwhile, under William I and Otto Van Bismarck, Prussia sought to expand its influence within the German states (which at this time still constituted a Confederation rather than a unified country). By winning the Austro-Prussian War, Prussia was able to consolidate its claim as German hegemon, resulting in a new confederation under Prussian leadership. As Prussia became more powerful, it also became more threatening to French political interests and national security.
The trigger for the Franco-Prussian War came when Spanish Queen Isabella II was deposed, with Hohenzollern Prince Leopold (of the same dynasty ruling Prussia) one of the claimants to the throne. The idea of a Hohenzollern ruling Spain was a threat France could not afford to let pass, as it would have risked being caught between Prussia on one side and Spain on the other, a potential crisis in the making. Bismarck, already looking for any excuse to fight France on his own terms, was able to use this diplomatic crisis to draw France into a disadvantageous war.
From a purely military angle, the Franco-Prussian War was a disaster for the French, and Germany had already occupied parts of France before the French army had even been able to begin rallying a defense. However, less often discussed are the diplomatic maneuvers through which Bismarck was able to isolate the French diplomatically.
As mentioned earlier, in 1859 Napoleon III allied with Piedmont-Sardinia to wage war with Austria. Austria had not forgotten or forgiven this move. Napoleon III had likewise alienated his former ally in Italy. Meanwhile, in 1866, France attempted to pressure Prussia into allowing France to annex Luxembourg and Belgium, which Bismarck later publicized, thus ensuring British neutrality. Meanwhile, in 1863, Bismarck actively assisted the Russians in suppressing a revolt in Poland, giving him diplomatic goodwill from Russia. As a result, when war began between France and Prussia, France found itself without allies. What ensued was overwhelming military defeat, leading to the collapse of Napoleon III's Second Empire (which was quickly replaced by the Third Republic) on the one hand and the unification of Germany on the other.
Citation Note: in preparing this answer, I drew upon John Merriman, A History of Modern Europe: From the Renaissance to the Present (Third Edition). New York: W. W. Norton, 2010. Details concerning the rise of Prussia through to the Franco-Prussian War (including Bismarck's diplomatic maneuvering to isolate France) were found in pages 660–668. Likewise, information concerning the Second Empire (both economic and diplomatic/political in theme) spanning from Louis Napoleon's coup d'etat in December, 1851, through to the Franco-Prussian War could be found in pages 725–731. It is in the latter of these two sections that one finds French concerns vis-a-vis the Spanish monarchy as well as a more detailed account of the war itself.