For history essays it is easy to throw in a lot of dates and quotes and feel as if you are backing up your argument effectively because you have provided "evidence". You must ensure however, that your evidence is fully explained and genuinely supports the point you are making rather than just including it. If it is not actively enhancing your point is actually more detrimental to your argument.
When you make a statement, make sure that within your essay the reader can understand why you made that statement and why it is correct. When reading through your essays it is often helpful to question every point you make with why? and how? If the answers to these questions are within your essay you can see that your argument is well supported. Use dates and historical evidence in conjunction with this but do not rely on them.