That's not correct. Many slippery slope arguments are perfectly valid and sound, namely whenever there is plausible evidence for the existence of the respective slippery slope. It can for instance be based on historical precedents, or on probabilistic or convincing balance of consideration sub-arguments for the slippery slope premise.
Whether its a fallacy or not depends a lot on how warranted the slippery slope premise is (notwithstanding other, possible logical errors that might me made in setting up the argument, of course).
Wars are sometimes caused by small acts of violence (e.g. at the border) that are mutually retributed, becoming larger and larger, until the hostilities turn into an all out war. This is sometimes called "escalation of violence." If wars are sometimes caused in that way, then there must be some wars for which a slippery slope argument concerning the escalation of violence is not fallacious.
Whether its a fallacy or not depends a lot on how warranted the slippery slope premise is (notwithstanding other, possible logical errors that might me made in setting up the argument, of course).