As others have already stated 18th & 19th century states were just as likely to use pre-emptive or surprise attacks as any modern state. There were often efforts to time the official declaration of war so that it would arrive just before the troops began to move or cross the border, so one could not accuse the aggressor of violating the laws of war, while still surprising the enemy. Or as Prussia did in 1806, the troops might move before any formal declaration of war. Sometimes you could see things coming as the offending party would start to remove its diplomatic personnel or citizens from the state they were planning to go to war with. One would then withdraw the ambassador from said country after presenting the declaration of war, though often a diplomat of lower rank would remain at the embassy. After all, you needed someone for the enemy to surrender to...

-Alex