Most people who pursue a second citizenship stop reading once the document arrives. You meet the requirements, make the investment or prove the ancestry, and the deal feels done. What follows that moment is where most of the complications begin.

The obligations attached to citizenship arrive by operation of law, often silently, and can reach your children as readily as they reach you. Several of the most consequential rules only become visible once the window to act has already closed.

Descent claims have deadlines

Whatever the strength of a bloodline claim, the window to act on it can close long before anyone thinks to check. In practice, waiting can extinguish the entitlement entirely.

Italy’s Law 74/2025 provides the sharpest recent illustration. Only descendants who had filed an application with a consulate, received notice of an appointment, or had a pending court claim by 11:59 p.m. Rome time on 27 March 2025 retained access to the old broad route through an Italian ancestor. The Constitutional Court upheld the reform in March 2026, and a reference to the Court of Justice of the European Union, lodged in July 2026, will take at least a year to resolve.

Ireland carries the same lesson through different mechanics. Citizenship through a grandparent flows from the date your birth is entered on the Foreign Births Register, not from your birth itself. A parent who delays registration can permanently lock a child out of the entitlement, given that registration currently takes around a year to complete.

Generation limits shift more often than most people realise

Where a bloodline claim runs out depends entirely on which country you’re looking at, and the answer changes more often than most applicants expect. Italy now recognises claims primarily through a parent or grandparent who held only Italian nationality. Ireland extends to a grandparent, and to a great-grandparent only if the intervening parent registered first. Slovakia and Bulgaria reach the third generation through facilitated naturalisation rather than automatic recognition, with Slovakia in July 2026 removing the residence condition for descendants of Czechoslovak citizens born on its territory. Canada removed its first-generation cap in December 2025, though children born abroad past the first generation qualify only if the Canadian parent spent at least 1,095 days in Canada before the birth.

Naturalisation in one country can cost you the passport you already hold

Not every country permits you to hold two passports, and some will remove the one you already have without waiting for you to act.

Austria requires release of an existing citizenship before naturalisation completes, unless advance permission is granted. India and Singapore do not recognise dual nationality at all. Under Article 9 of China’s Nationality Law, a Chinese national who voluntarily acquires a foreign nationality loses Chinese nationality at that moment, with no application required and no notice given. Japan’s rule works the same way. The legal loss can sit undetected for years, surfacing when a passport comes up for renewal or when a child tries to inherit the status.

Military service follows the bloodline

What many families discover only after the fact is that a military liability can travel with a citizenship, reaching sons who have never set foot in the country.

For South Korean males, the liability extends to dual nationals born and raised abroad. The option to renounce Korean citizenship closes on 31 March of the year a male dual national turns 18. Missing that date generally means waiting until service is complete or an exemption applies, which can stretch into his late thirties.

Greece tightened its own rules further under Law 5265/2026, in force from January 2026, narrowing the criteria under which a Greek man qualifies as permanently resident abroad and defers service indefinitely. Turkey and Israel also conscript dual nationals in certain circumstances.

Tax obligations that cross borders

Among the world’s tax systems, only two tie their core income liability to citizenship rather than residence: the United States and Eritrea. An American living permanently abroad still owes annual US filings, and the Foreign Account Tax Compliance Act requires foreign banks to report accounts held by US persons. Many so-called accidental Americans encounter this obligation only when a foreign bank raises the question.

Leaving the United States behind does not end the tax relationship cleanly. Someone classified as a covered expatriate, broadly a person with a net worth of at least USD 2 million or whose average annual income tax over the prior five years exceeded the relevant threshold, is treated as having sold all worldwide assets the day before renunciation. The first USD 910,000 of gain is excluded for 2026, and the balance is taxed. A separate rule taxes gifts and bequests from a covered expatriate to US citizens or residents at 40%, falling on the American recipient, under regulations that took effect in January 2025 with first returns due in July 2027.

Canada applies a departure tax when you cease to be a tax resident, treating most property as sold at fair market value at that point, a trigger that can arrive when you emigrate, not when you actually sell.

Citizenship can quietly expire without formal renunciation

Some countries condition citizenship on a genuine connection, and prolonged absence can eventually satisfy neither side of that requirement.

Under Denmark’s Nationality Act, a Dane born abroad who has never lived in Denmark and demonstrated no real tie to the country loses citizenship automatically at 22, unless statelessness would result. Retention requires either living in Denmark for three consecutive months, spending a year there before 22, or filing a formal application between the ages of 20 and 22. Once the deadline passes, there is no retrieval mechanism.

Investor citizenships can be revoked

Purchased nationality sits under closer scrutiny than most buyers anticipated when they applied. The Court of Justice of the European Union ruled in April 2025, in Commission v Malta, that Union citizenship cannot result from a commercial transaction. Malta repealed its programme three months later.

Cyprus has since moved to revoke citizenship from several hundred people naturalised under its own closed scheme, including family members of original investors. The ruling was subsequently cited by Italy’s Constitutional Court when upholding the descent reform, and by Italy’s own state lawyers arguing that the old broad bloodline regime would itself have conflicted with EU law.

Every one of these rules was already in force before the people it caught knew it applied to them. Understanding the landscape before you apply, or before a deadline expires, is the only point at which the options remain open.