The French Revolution affected other countries because there was immediately a perceived threat that the French would attempt to spread the Revolution to other countries. The movement to overthrow, or at least to limit, the power of the monarchy was, at the time, thought of in similar terms to the...
The French Revolution affected other countries because there was immediately a perceived threat that the French would attempt to spread the Revolution to other countries. The movement to overthrow, or at least to limit, the power of the monarchy was, at the time, thought of in similar terms to the perception of communism 150 years later: namely, that this was a plan to take over the world and to convert countries to the same anti-monarchical form of government the Jacobins in France were attempting to set up.
Virtually all of the states of Europe in 1789 were monarchies, and the only large country that had a degree of genuine democracy as a constitutional monarchy was Great Britain. Given that egalitarian principles had been expressed again and again throughout the eighteenth century by Enlightenment philosophers and political theorists, and given the example of democratic government which had begun just a few years earlier in America, it was understandable that European kings and queens would become more than a bit paranoid over what was occurring in France. In addition, there was the personal connections of Marie Antoinette, as she was the sister of the Holy Roman Emperor Joseph II (and Leopold II after Joseph's death in 1790).
Yet it was not until 1792 that hostilities broke out, with Austria and Prussia first declaring war on Revolutionary France. As long as no actual harm had come to Louis XVI and Marie Antoinette, the other powers had taken no overt action. The first decisive change occurred in June 1791, when the king and queen attempted to escape across the frontier into Germany and were halted and then placed under what was essentially house arrest. It was clear that the king was now a prisoner in his own country. From this point, the series of events including the king's being forced to accept the constitution in September of 1791, the abolition of the monarchy a year later, and the execution of both the king and queen in 1793 meant that a state of irreconcilable conflict existed between France and the other European states. What followed, of course, was over 20 years of nearly continuous war until the final defeat of Napoleon in 1815 at Waterloo.