While realism and liberalism have some similarities, their differences are much more important.
Both realism and liberalism believe that the world is a dangerous place. Both realize that countries can go to war with one another and destroy one another. Both realize that there is no world government in place that can prevent countries from doing harm to one another.
Both realism and liberalism believe that military power is important. Both sides understand that states can use military power to get what they want. They are both aware that countries without military power can be abused by other countries.
While these are important similarities, they are less important than the differences. First, realists believe that military power is the only relevant kind of power. Realists do not believe that things like “soft power” can be at all important. By contrast, liberals believe that military power is only one kind of power and that it is not even the most important kind of power in many cases. Instead, they believe that things like economic power and moral power can be more important than military might.
Second, realists believe that there is no point to international institutions because they cannot force countries to comply. They believe, for example, that the United Nations is pointless because it cannot keep a country from doing what it wants (say, for example, that it cannot prevent Russia from annexing the Crimea). Liberals understand that the UN cannot force countries to obey, but they believe that it is still very important. They believe that international organizations give countries ways in which to cooperate with one another and to gain one another’s trust.
The last major difference between the two that I will address here is the two theories’ different views on what states want. Realists argue that all states have the same interest. All states are interested in increasing their power in the world. Liberals do not agree. They say that there are many states that really do not try to increase their power (remember that power is only defined for realists in military terms) and that we cannot understand other states unless we understand what it is that they want.
These differences (along with many others) greatly outweigh the similarities between these two theories of international relations.