What will be the tipping point for national legal recognition of gay marriage? Politicians, scholars, activists, and historians can all disagree but one thing is certain: the Supreme Court cannot avoid at least the first significant step toward that tipping point this month.

Two gay marriage cases are before the Court — one that challenges California’s gay marriage ban and one that challenges the federal Defense of Marriage Act (DOMA) statute. Regardless of how the Court rules in California’s Hollingsworth v. Perry case, in U.S. v. Windsor the Court must strike down the provisions of DOMA that forbid federal recognition of state licensed gay marriages. To do otherwise would ignore the long history of U.S. marriage law.

Prior to the passage of DOMA in 1996, any marriage recognized by a state was recognized by the federal government, even if that marriage was not recognized by one or more other states — as was the case with interracial marriage before 1967. Yet today DOMA forbids the federal government from recognizing same-sex marriages from the 12 states (and the District of Columbia) where gay marriage is legal. This refusal to honor a state-issued marriage license is unprecedented, and contradicts more than two centuries of federal deference to states’ rights to regulate marriage and cannot stand.

The Supreme Court’s DOMA decision will immediately impact only couples in states that currently issue marriage licenses to gay couples. Nonetheless, this simple, unavoidable decision will prove, in hindsight, to be the watershed moment in the inexorable march toward nationwide recognition of gay marriage. Here is why.

As soon as the Supreme Court forces the federal government to recognize the marriage licenses of gay couples no differently than it already does marriage licenses of straight couples, gay couples in all seven states that currently offer either civil unions (four) or domestic partnerships (three) instead of marriage will immediately have a strong Fourteenth Amendment Equal Protection violation claim. After all, the entire premise of civil unions is that they offer a package of rights identical to those of marriage. While this may be true within any individual state, the moment that the federal government begins to acknowledge gay marriages while continuing not to grant the over 1,000 federal rights that come with marriage to any couple in a civil union or domestic partnership, there is a clear case of disparate treatment.

Any state that then refuses to change its civil unions or domestic partnerships to full marriage licenses will face a legal challenge that the Supreme Court will have to take on appeal. The Court will then have to rule that states whose courts have held that their state constitutions require equal treatment of gay and straight couples wishing to legally commit, must do so by issuing full marriage licenses to both. To do otherwise would result in having gay couples in those states deprived of the myriad of marriage rights granted to the states’ straight couples by the federal government.

Of course, 30 states have state constitutional amendments banning gay marriage. Given the entrenched opposition to voluntary change on this matter in certain regions of the U.S., the Supreme Court will very likely have to rule one day in the not too distant future on all state laws prohibiting gay marriage. It is possible, but highly unlikely given the Court’s propensity to rule as narrowly as possible on any given case before it, that the Court will do so this month in the Hollingsworth case. Even if the Hollingsworth decision is limited to California, however, once there are 19 states granting marriage licenses to gay couples and all of these are being honored by the federal government, it will become increasingly difficult to ignore the denial of civil marriage rights to gay couples in 60 percent of the states and the time will come for the ultimate Supreme Court case on gay marriage.

With the federal government granting marriage rights to gay couples and forcing states to upgrade from civil unions to full marriage, the Court will no longer be able to postpone resolution of the issue of whether the state constitutional amendments themselves violate the federal Fourteenth Amendment by failing to provide an adequate legal reason for treating same sex couples seeking civil marriage differently than opposite sex couples seeking the same. The Court ruled exactly this way in Loving v. Virginia in 1967 when it struck down state laws prohibiting interracial marriage as unconstitutional since they too failed to provide an adequate reason for treating the forbidden couples any differently than the acceptable ones. The Court will eventually rule likewise on state laws banning gay marriage. On the historic day when the court makes this ultimate ruling, we will all look back to the day this month when the Court takes its first small but irreversible step in that direction, by forcing federal recognition of the gay marriage licenses that have already been granted.

Marianne DelPo Kulow is associate professor of Law, Taxation and Financial Planning at Bentley University.