Romans 15:33 calls God a God of peace, but Exodus 15:3 calls Him a man of war.
"The Lord is a man of war; the Lord is His name." (Exodus 15:3)
"Now the God of peace be with you all. Amen." (Romans 15:33)
The most basic answer to this supposed contradiction is that both statements are true depending on the time and context. With that in mind, let's examine a couple different ways we can look at this.
First, consider the fact that war is often fought for the purpose of bringing about peace. During World War II, Europe and Asia were engulfed in fighting. Sitting on the sideline was the United States, hoping that this situation would be resolved without our involvement. The U.S. wanted peace, not war.
On December 7, 1941, Japanese pilots made a surprise attack on Pearl Harbor that launched the U.S. into war against Japan. That prompted Hitler to declare war on the US, bringing the country into both fronts. The U.S. became a nation of war in an effort to be a nation of peace and to help bring peace to the world at that time.
In the same way, God is a God of peace. He created an originally perfect, peaceful world that has been subjected to war only because of mankind's rebellion. Of course, God knew this, had a plan for victory, and offers a time when peace will reign for eternity. However, bringing about this peace often includes the necessity of war.