War Cadence — ruling 1

The ability only applies to blocks declared after it resolves. It will not add costs to any blockers already announced.

War Cadence — ruling 2

You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t block unless the controller chooses to pay the cost (a cost of zero is not automatically paid). You can use this to make your creatures unable to block an attacker.

War Cadence : règles, fonctionnement et interactions (MTG)

War Cadence is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: This turn, creatures can't block unless their controller pays for each blocking creature they control.

How it works : War Cadence

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
The ability only applies to blocks declared after it resolves. It will not add costs to any blockers already announced.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t block unless the controller chooses to pay the cost (a cost of zero is not automatically paid). You can use this to make your creatures unable to block an attacker.
No other card is named in this ruling.