A credible foreign policy is one where nobody has to guess what your country will do in a given circumstance. That could be very good, or very bad. For instance, under Jimmy Carter, we were pretty sure how the U.S. would respond to any provocation or challenge-cave. The Romans, on the other hand, had a very credible foreign policy: mess with us, you die. The followed this policy religiously, and as a result, soon they did not have to employ it all that often. In the 19th century, the British had a similar approach: mess with us, and soon your country joins the other pink splotches on the map. Pretty soon, they didn’t have to do it very much, either (of course, by that time, they had already taken most of the good stuff).
It’s inconsistency that leads to confusion and miscalculation which in turn leads to war. The U.S. is famous for this-making noises about defending China in the 1930s without bothering to accrue the military power needed to deter Japan, and then putting the Pacific Fleet (the main deterrent force) in an exposed position at Pearl Harbor (this is called leading with your chin). You would think that we might have learned our lesson, but only five years after World War II we are sending mixed signals regarding Korea. A decade later, and we are sending mixed signals on Cuba and Vietnam. In 1990, we sent mixed signals on Kuwait. In each instance, we said one thing, meant another, and our enemies of course chose to take us at our word. In the end, they paid for it, but so did we.