Having your enemy underestimate you is never a problem.
Said another way: "Wars begin when two sides disagree about the fundamental balance of power between them. Wars end when both sides agree on the fundamental balance of power between them."
Look how hard liberals attack when they feel their "rights" such as closed-shop laws are changed, look how fast they flooded the state capitals in Wisconsin, Indiana, and Michigan. They are organized in a way that we aren't. They have deep pockets. They could call on international support and probably get it.
They don't underestimate so much as misestmate us. (I don't think misestimate is a word, but I'm using it to describe an inaccurate assessment caused by false reading of data, as opposed to false data.)