On Feb. 19, Capital One (COF 0.71%) announced it would acquire Discover Financial Services (DFS 1.37%) in an all-stock transaction valued at over $35 billion. The move could make Capital One the largest credit card issuer in the U.S. and create greater competition for Visa (V -0.05%) and Mastercard (MA 0.39%), whose stocks fell following the news announcement. Here's what it could mean for investors in those companies.Capital One could become the largest card issuer in the U.S.Capital One provides its custo ...