Anxiety in children tends to look different than in adults. Children will behave with resistance, which appears like disobedience, when they are feeling overwhelmed and powerless. As adults, it is important to remember that whenever a child appears defiant, they are actually telling us through their behaviors that they don't know how to deal with their emotions.
Sometimes, we don't have the best responses, especially when we are frustrated or even angry at them. But a more effective way to handle a difficult situation is to help them practice calming down and then addressing the real cause to their anxiety. This process is not easy, but necessary if we are to teach our children how to best handle their emotions and carry those skills to adulthood.
Remember that children usually don't know how to appropriately express their needs, so it is up to parents (and other adults in their lives) to figure out what the child is saying through their behaviors. Parents will find it easier to deal with a defiant child when they view the behavior as a signal for help instead of a personal offense.
A great way to prepare for such occassions is to practice when the child is already calm and willing to learn. This will allow everyone involved to feel more in control of their own behaviors; a goal we all want to meet!